Mythology is filled with part-this, part-that creatures. A chimera has the head of a lion and the body of a goat. A mermaid is half fish, half hot babe. On "South Park," Al Gore dreamt up the fearsome ManBearPig. The 2009 Mercedes-Benz CLS-Class follows in this tradition: a "four-door coupe" that blends the sleek lines of a luxury two-door with the practicality of a sedan. Thankfully, there's nothing mythical about it.

There are two versions of the CLS: the CLS550 and the CLS63 AMG. For the CLS550, Mercedes takes its 382-horsepower, 5.5-liter V8 and adds it to a stretched E-Class chassis. Also part of the plan is swoopy, coupelike exterior styling and the latest technology, such as an Airmatic semi-active suspension. Airmatic automatically diminishes the dive, pitch and roll expected of luxury sedans on tight, twisty roads -- making the CLS nearly as fun to drive as a smaller coupe.

Not to be outdone, in-house tuner AMG takes the CLS550 and gives it the "Oh yeah, well, watch this!" treatment, creating the CLS63 AMG. The already powerful 5.5-liter engine is replaced by a 6.2-liter V8 that produces a ludicrous 507 hp, fed through a seven-speed AMG SpeedShift automatic transmission. This tranny features steering-wheel-mounted paddles and impressively swift gearchanges with automatic throttle blips on downshifts. AMG-optimized springs and shocks, larger brakes, a meaner exhaust and more aggressive tires round out the upgrades from the AMG madmen.

Naturally, the compromise between coupe and luxury sedan has created some shortcomings, albeit minor ones. A consequence of the sexy arched profile is reduced outward visibility and rear headroom, while the less-than-sedan-sized doors make the passenger/pedestrian transition slightly more difficult. Somehow we doubt that those smitten by the shape of the 2009 Mercedes-Benz CLS-Class will be bothered by such details.

Since the CLS founded the oxymoronic four-door-coupe segment in 2005, it has yet to be challenged by a direct competitor. Some may argue the CLS550 and CLS63 AMG aren't quite sure what they want to be, but like mermaids and ManBearPigs, it's hard to deny that this bodacious Benz is a pretty cool creation.

The 2009 Mercedes-Benz CLS-Class luxury sedan is available in two versions: CLS550 and CLS63 AMG. Standard appointments for the CLS550 include 18-inch wheels, an electronically adjustable air suspension, a sunroof, leather upholstery, a tilt/telescoping steering wheel, four-zone automatic climate control and a Harman Kardon audio system with a CD changer, an auxiliary audio jack and satellite radio. In addition to its various performance upgrades, the CLS63 AMG is outfitted with 19-inch wheels, front and rear aprons, side skirts, a rear spoiler, stickier tires, heated front seats, enhanced leather with Alcantara inserts and a racier steering wheel.

The CLS550 can be equipped with three option packages. The $4,760 Premium package includes adaptive headlights and washers, electronic trunk closure, heated/ventilated seats, iPod and MP3 access, keyless ignition and a rear sunshade. The $920 Trim Package adds premium wood accents and an upgraded leather interior. For those wanting the AMG look without the over-the-top performance, the $5,090 AMG Sport Package comes with AMG exterior styling cues, special five-spoke wheels, a sport steering wheel and paddle shifters. Also available are Parktronic sensors and Distronic Cruise Control.

For the CLS63 AMG, the $9,000 AMG Performance Package increases top speed to 186 mph from 155 mph and includes high-performance tires, sterling-silver-finished wheels, even sportier suspension tuning and a choice of carbon-fiber or wood interior trim.

Both CLS models are V8-powered and controlled via seven-speed automatic gearboxes, but that's where the similarities end. The CLS550's 5.5 liters unleash 382 horses, while the 6.2-liter CLS63 AMG churns out a whopping 507 hp. The AMG version leaps to 60 mph in 4.3 seconds -- on par with the Audi R8 and Lamborghini Gallardo, except two additional passengers can get in on the fun. The CLS550 hits 60 mph in 5.4 seconds, which is still sports-car quick.

The 2009 Mercedes-Benz CLS-Class comes with antilock brakes, stability control, active front head restraints and enough airbags to make the interior resemble a bouncy castle at a kid's party. For good measure, when the sensors predict an impending crunch, the sunroof closes, the seatbelts cinch tight and the passenger seat positions itself for optimal airbag protection.

Drop either CLS engine under any hood and you're pretty much guaranteed a few grins. Mount them in a competent platform, as they are in this case, and the grin factor skyrockets. The Airmatic adjustable suspension gives the driver the option of a Comfort mode as well as two distinct Sport modes, and for the well-heeled adrenaline junkie, the CLS63 AMG delivers performance usually reserved for exotics with half as many doors and seats.

Because of its low roof line, the 2009 Mercedes CLS-Class may prove problematic for taller rear passengers. However, the abundance of shoulder room and legroom should make up for the low ceiling. For those of average height and build, the rear seats are comfortable and spacious.

Most interior functions are controlled via Mercedes' familiar COMAND interface. While the current system is an improvement over previous versions, thanks to mild revisions such as the inclusion of voice-activated functionality, COMAND seems cumbersome when compared to other such systems from rival automakers.