The standard Airmatic suspension in the S430 and S500 uses compressed air in bellows-like springs at each corner to maintain constant ride height regardless of load. On the highway, the car automatically lowers itself by about an inch, which improves aerodynamics and thus fuel mileage. Airmatic features adaptive damping, which allows the driver to adjust the shock absorbers according to road conditions, load and speed, from softer to firmer. Or the suspension will do this automatically. This system is not the same as the more complex Active Body Control suspension, which is optional on the S500 and S430 and standard on the S600 and S55 AMG, and virtually eliminates body roll in cornering, squat under acceleration, and dive during braking. The Airmatic suspension has three settings. One tester thought the firm setting was quite livable and appropriate for all situations, while another preferred the soft and middle settings around town.

As expected, the S-Class is quiet, really quiet, when it's blasting along at high speeds. But the Airmatic suspension isn't perfect. It rocked over shallow holes in the road at 10 mph, and at the speed limit we could feel it lift from side to side over rough or grooved surfaces. We could also feel a jiggling in the steering wheel over bumps on an otherwise smooth road.

Underway, the S-Class engines are quiet, relaxed and so smooth that they give no hint of operation. At idle, however, we found both the S430 and S500 engines a bit rougher and noisier than expected.

The S430, the most popular of the S-Class sedans, has plenty of power. Nail it at 65 mph and it accelerates smoothly around the offending vehicle. It cruises easily at high speeds on cross-country trips and it's responsive around town. We never felt like we were missing something by not having the larger S500 engine. Until we drove the S500. Then it's clear that the S430 does not offer the same rocket-like response. Perhaps only certified lead-foots will notice.

We are among them. Drop the hammer on the S500 and the V8 makes neat sounds. The car is most fun when it's using its torque and growling. That's when a big, elegant luxury car can feel like a hot rod. The specs say the S500 engine produces its full 339 foot-pounds of torque between 2700 and 4250 rpm, but it didn't feel like the V8 hit its sweet spot until nearly 4000 rpm. When you floor it at 3000 rpm, it kicks down a gear or two, as if to get more power when it should theoretically already be in the right gear for maximum torque. Either way, the S500 is exceptionally smooth and quiet as the revs increase into the 5000-rpm range, so it's easy to hit the 6000-rpm rev limiter in second or third gears when you're in the manual-shift mode.

For 2004, Mercedes has equipped the S430 and S500 with a passenger-car first: a seven-speed automatic transmission. All those gears allow the trans to maintain closer to an optimal gear ratio in any given driving situation, while at the same time offering a larger ratio spread between the lowest and highest gear. The electronic controls have more flexibility to adjust shifting to achieve optimal fuel efficiency. The seven-speed automatic lowers average engine speed, which reduces fuel consumption and minimizes noise.

The seven-speed automatic once again demonstrates Mercedes' continuous quest for the ultimate automotive technology, which is something buyers spending $80,000 for a sedan have a right to expect. Ultimately, this transmission improves response, acceleration and fuel economy. But like many systems in the S-Class, it sometimes wants to be the boss when the driver has different expectations. Sometimes it will shift up when we expect it to hold; other times it wants to shift down twice, when we're expecting once. It may take some time for many of us to get used to it, or some time for the transmission to get used to us.

The S600 and S55 AMG are rocket sleds, plain and simple. It's difficult to convey the sensation of planting your foot to the floor on a long, open straight in either of these big sedans, except to say that, with the amount of mass the engines are moving at an amazing rate, it feels like a commercial airliner as it approaches take-off speed. Either will accelerate to 60 mph faster than most sports cars or sport coupes, and between the two acceleration is basically a wash.

So which of these big-kid toys would we choose? The V12 S600 is silky smooth, with muted turbochargers and more conservative, understated looks. The S55 is a bit rougher, by design. Its supercharged V8 vibrates more obviously, and its aggressive body cladding screams speed. Even if money were no object (the S600 starts at $12,500 more), we'd probably take the S55, simply because its ABC active suspension is calibrated to limit body roll in corners more aggressively. Scratch that. We'd take the S600 with the sport package, minus the body kit.

Yet even in the S430, there's enough power that, on a wet freeway with the cruise control set at 72 mph, the traction control got a workout. Whenever the tires hydroplaned in puddles that formed in the freeway grooves, you could feel the wheels spinning and biting, spinning and biting. It was interesting to blast through them like this, with no feet on the pedals. A few times the cruise control deactivated because the brakes were automatically dabbed.

That traction control made us a little nervous once, as we pulled onto a two-lane from the side of the road. A truck suddenly came barreling over the hill at us, and we spun a little gravel to get the heck out of there. Problem was our traction was still being controlled, even after our rear wheels were on pavement. For whatever reason, the car didn't believe we should be accelerating just yet, so it wasn't letting us, using its powers of throttle intervention. The car was wrong. The lag was unwanted and unneeded, and it made us think: Traction control doesn't always get you away faster. It just gets you away without spinning your wheels. And sometimes faster is safer, even if it involves a little wheelspin.

We battled with the brain in the anti-lock brake system, too. We made a panic stop at 60 mph to test the ABS, and when we lifted off the pedal the brakes stayed applied for another beat. Yep, we got Brake Assist. It would have been nice if we had asked for it. There's a brake release switch that takes a split second to activate, as a Mercedes engineer later explained to us. Around town, the brakes are smooth, powerful and easy to modulate for even, progressive stops.