The 3.6-liter V6 with direct-injection developing 302 horsepower is standard equipment, with a 320-hp 4.6-liter V8 optional.Both STS models come with a six-speed automatic transmission.Both are modern aluminum powerplants boasting double overhead camshafts, four valves per cylinder and variable valve timing.

Safety technologies include Cadillac's Lane Departure Warning and Side Blind Zone Alert systems, plus a sophisticated StabiliTrak electronic stability control system that uses steering and other inputs, and brake and throttle controls, to reduce the chance of a dangerous skid.

The roomy cabin is superb, with comfortable, supportive seats that are infinitely adjustable, ample storage space, and superior sound systems, including 15-speaker Surround Sound.

The STS is unmistakably a Cadillac, but it's also a four-door performance sedan for people who like to drive.The standard suspension is tuned for a smooth and comfortable ride, while the available active suspension instantly adjusts to any driving situation, whether cruising through a sea of potholes or accelerating around a sweeping turn.Rear-wheel drive provides the balance and control serious driving enthusiasts demand, while the available all-wheel drive very beneficial in bad weather.

Changes for 2010 are minor, with two new colors: Vanilla Latte and Tuscan Bronze ChromaFlair.

Standard STS features include leather upholstery, aluminum interior trim, dual-zone automatic climate control, Bose AM/FM/CD stereo, interior auto-dimming rearview mirror, OnStar telematics (with one year's service), StabiliTrak electronic stability control, automatic high-beam control (Intellibeam) for the headlights, rear park assist and the usual lineup of power assists, creature comforts, extra conveniences and driver aids.

Optional packages for the V6 model include the V6 Luxury Package ($3,540) with two-position memory for seats, mirrors, climate and radio controls; Rainsense wipers; six-disc in-dash CD changer; Sapele wood trim on the instrument panel, steering wheel, console and center stack; heated and ventilated front seats with four-way power lumbar; heated rear seats; heated steering wheel; tire-pressure monitor, universal home remote; and a cargo net.Other packages include the V6 Luxury Collection ($4,500), a V6 Luxury Level 2 Package ($3,950), a V6 Premium Luxury Collection ($2,740), a V6 Platinum Edition ($7,950), and a V6 Premium Package ($9,100).Options for the V8 include the V8 Premium Luxury Collection ($4,655), V8 Platinum Edition ($6,500), and V8 Performance Package ($12,440).

Stand-alone options, included in some of the packages above, include Performance Handling ($1,600), power moonroof ($1,200), Performance Brakes ($395), DVD rear entertainment ($1,985), 18-inch polished alloy wheels ($600), and enhanced StabiliTrak with active steering for the V8 ($995).A Bose 5.1 Studio Surround Sound system with six-disc CD changer and DVD-based navigation is available.

Safety features that come standard on all models include dual two-stage frontal airbags, front seat-mounted side airbags, front-and-rear side-curtain airbags, anti-lock brakes, StabiliTrak electronic stability control, tire-pressure monitoring, and OnStar; its operators will contact appropriate assistance if there is an airbag deployment (meaning the vehicle has been in a wreck), and the passengers don't respond to calls.

Lane Departure Warning uses a camera located between the inside rearview mirror and the windshield to detect lane markings on the road and alert the driver when the vehicle inadvertently strays from the intended lane.Side Blind Zone Alert uses radar sensors behind the rear fascia that signal an audible and visual alert if objects are detected where it might be difficult for the driver to see them.Enhanced StabiliTrak with active steering is available only on V8s with AWD.

Up front the STS has a boldly traditional eggcrate grille with Cadillac's trademark vertically stacked headlamps.Chromed air extractor vents mark the clean flanks, and more subtle chrome accents adorn the door handles and rocker flares.

The rear has vertical taillights bracketing a tall, squared-off trunk lid.Recessed into the trunk's rear surface is a trapezoidal inset, big enough for European-spec license plates, housing large backup lights at the left and right extremes.Dual polished aluminum exhaust tips exit below the rear bumper.

The Platinum Edition features 18-inch chrome-finish wheels, a unique grille finish, Olive Ash wood trim with Saddle-stain leather-wrapped interior, premium floor mats, Tuscany full-leather seats and additional features.

The seats in all models are refreshingly supportive, for a Cadillac, without being overly firm.All essential controls are within easy reach, although there could be more clearance between the lower door panels and seat bottoms to access the front seat adjusters.For this reason, we especially liked the seat memory feature, which often saved us from having to reach down for the adjusters.The interior is noticeably roomier than that of the marginally smaller CTS.

Instruments are easily-scanned, white-on-black round analog gauges, with a large nested tachometer and speedometer between the smaller fuel and engine temperature gauges.The speedometer changes between English and metric electronically, so there's only one set of numbers around its circumference.

Cruise control and running lights are managed via a stalk on the left side of the steering column, windshield wipers and washers with a stalk on the right.Buttons in the steering wheel spokes provide redundant controls for audio and driver information functions.The steering wheel hub looks substantial, and even a bit futuristic.The available head-up display projects speed and other key information onto the windshield, so that the driver does not need to look down at the gauges.

The available Bose 5.1 stereo incorporates Surround Sound with 15 speakers.An integrated six-disc CD/DVD changer plays DVD-A, CD, and MP3 formats.An eight-inch VGA-quality touch screen and advanced navigation, Bluetooth phone capability, and OnStar with advanced voice recognition are all part of the package.A word of caution, however: The top-level stereo system, although delivering superb Surround Sound, is multi-tasked with a navigation system that, in combination, demands study of the owner's manual to operate with any degree of alacrity and confidence.

OnStar features a fully integrated GPS navigation system called Turn-by-Turn Navigation.Turn-by-Turn allows drivers to talk to a live advisor, who can download complete step-by-step directions to the vehicle through the OnStar system.These audio directions then automatically play through the vehicle's stereo as needed (after the OnStar operator hangs up), triggered by the system's GPS capabilities.This enables drivers to be led to their destinations while keeping their hands on the wheel and eyes on the road.

Storage cubbies include map pockets in all four doors.The front center console is deep and wide and pre-wired for cellular and Bluetooth.The glove box, though, is barely sufficient to hold the navigation DVD case and owner's manual.Two cup holders are provided front and rear.The trunk is fully lined, with articulated, gas-pressurized struts.

Fit and finish are top grade, with notably tight trim tolerances.Careful attention was paid to reducing noise, vibration and harshness, with remarkable and commendable success.Specially laminated windshield and front door glass, wind tunnel-tuned outside mirrors and high-density/low-mass sound-deadening padding make this a quiet car even by Cadillac standards.

The Platinum Edition features hand-cut and hand-sewn leather on the instrument panel, door trim and center console areas.The seats are fully covered with a semi-aniline leather called Tuscany.Wood trim is an Olive Ash burl stained a saddle shade.Chrome accents highlight the instrument panel and doors.Even the floor mats are upgraded, and there are Platinum badges on the door sills.Only two interior colors are offered: Ebony or Cashmere with Cocoa accents.

The V6 delivers 302 horsepower at 6300 rpm and 272 pound-feet of torque at 5200 rpm.It delivers good fuel efficiency, while its direct injection technology enables a 25-percent reduction in cold-start hydrocarbon emissions.With direct injection, fuel is sprayed directly into the cylinder, rather than into the intake port.As the fuel vaporizes in the cylinder, the air and fuel mixture is cooled.This enables the use of a higher, 11.3:1 compression ratio for better thermodynamic efficiency.The 3.6-liter V6 also features an isolated fuel-injector system, which helps reduce engine noise.

The six-speed automatic that comes on all models features a generous 13.88:1 maximum overall first-gear ratio for rapid launching off the line (13.11:1 with AWD), while also providing two overdrive ratios that decrease engine rpm and reduce noise levels while cruising at highway speeds.

The V8 generates 320 horsepower at 6400 rpm, and 315 pound-feet of torque at 4400 rpm.Both the 3.6-liter V6 and 4.6-liter V8 use electronic throttle control, to match the engine's performance to a variety of driver demands, from sedate highway cruising to rambunctious back-road motoring.

We especially enjoy the STS V8 with the optional performance handling package.Nudge the shift lever over to the right, into the manu-matic gate, and the selected gear will hold all the way up to the engine's redline, and this car can be taken to extremes heretofore beyond the reach of sedans wearing the Cadillac wreath and crest.The suspension engineers have demonstrated they understand the difference between too stiff and responsively firm.Thankfully, all the sound filtering and deadening doesn't keep the V8's throaty exhaust note out of the cabin.

The all-wheel-drive system is a hard package to top, complemented by Magnetic Ride Control and the latest generation StabiliTrak, though this adds some weight.Body lean in even the tightest switchbacks is minimal and mild dips are well controlled.Biasing 60 percent of the power to the rear wheels gives the all-wheel-drive STS the sporty dynamics of rear-wheel drive while sending enough power to the front wheels to help in bad weather.It's a good setup.

The electronic steering is a delight, its only shortcoming a slight softness on-center.The STS tracks well into and through corners, especially with the 18-inch, low-profile tires.

Active steering, available exclusively on V8 models with AWD, is integrated into the StabiliTrak stability control electronics.In addition to controlling brakes and reducing engine power, StabiliTrak with active steering can turn the front wheels to reduce skidding when rear wheels lose traction.The system includes a new steering motor assembly combined with computer-driven electronic controls that measure wheel slippage at all four wheels independently during acceleration, braking and adverse road conditions.

Brakes on the STS are up to the car's potential, with a firm pedal and a feel that's reassuringly linear.All of the various performance packages include four-piston Brembo calipers, with quite large vented brake rotors both front and rear.

The Lane Departure Warning system engages above 35 mph.A green light indicates that the system is turned on and working.Should the driver cross a detected lane marking without signaling, the light flashes amber while an alarm beeps three times.The system is designed to alert the driver to take appropriate action to move the vehicle back into the correct lane.To avoid nuisance alerts, the system is designed to not provide an alert if the turn signal is on or if the driver makes a sharp maneuver.The driver can switch it off, in which case the light goes out entirely. The Side Blind Zone Alert system uses radar to sweep an 11-foot zone on either side of the vehicle; in other words, about one lane over.The zone starts at each side mirror and reaches back about 16 feet.With the system engaged, an amber symbol lights up in the outside mirror whenever another vehicle enters this blind zone.Cadillac cautions that the system is not designed to detect vehicles outside the side blind zone that may be rapidly approaching, or pedestrians, bicyclists or animals.It's designed to ignore stationary objects, such as fire hydrants or parked cars.In addition, the system displays do not come on while the vehicle is approaching or passing other vehicles.

The V6 and V8 engines represent the latest GM thinking, with double overhead cams, four valves per cylinder and variable valve timing.Called VVT, this system continuously varies valve operation to generate the most power from the least amount of fuel with the lowest emissions possible.Torque is what most of us experience as power in everyday driving; torque is what gets a car moving in the first place, as when accelerating from an intersection.Recognizing that, Cadillac engineers designed the STS engines to generate lots of torque throughout the rev range for responsive performance at all engine speeds.

NewCarTestDrive.com correspondent Tom Lankard reported on the STS, with Greg Brown reporting on the STS-V, and John F.Katz on styling.