The diesel engine represents the state of the art and we really like it. Available in most states, it's as clean as the ML's gasoline engines and gets much better fuel economy. With this engine, there is no black soot, no sulfur smell and no loud, clattering noise. At idle the diesel in the ML320 CDI is nearly as quiet as the gasoline engine in the ML350, and that's when you're standing outside next to the vehicle. Inside, a driver is hard pressed to tell any difference between the gas and diesel engines, measured by noise or vibration. The only obvious difference is that the diesel shifts to a higher gear under full-throttle acceleration sooner than the gas engine to take advantage of its torque and goes farther on a tank of fuel.

Mercedes reports identical 0-60 mph times of 7.9 seconds for the both the ML320 diesel and the ML350. Yet that doesn't tell the whole story, because in short bursts the diesel actually accelerates more quickly. Its whopping 398 pound-feet of torque. compared with 258 pound-feet for the ML350, comes across as a neck-whipping burst of acceleration, and the turbocharging maintains power in high-altitude environs. And for this the diesel buyer gets a fuel economy improvement of 30 percent or more in real-world driving. We're perfectly happy in the ML350, but if diesel were readily available on our appointed rounds, we would choose the ML320 CDI over the ML350.

The ML550 is more fun to drive. The 5.5-liter V8 engine will take this 2.5-ton SUV from 0-60 mph in less than 6 seconds flat, and then settles down to a background burble by the time you reach seventh-gear overdrive. The seven-speed automatic offers a manual-shift mode, and it's the best automatic Mercedes has built. It's flexible, in that it kicks down to a lower gear more quickly than its predecessors, and it almost always keeps the engine in the most productive part of its power band. Yet its overdrive top gear makes for quiet high-speed cruising and better fuel economy.

During several days of hard driving in mountainous, sinuous terrain, we found the ML550 a hoot. The suspension is good at minimizing body roll in high-speed corners, and it smoothes dips and potholes well. The optional AirMatic suspension we tested can be downright supple, which trucks aren't supposed to be. The larger standard 19-inch wheels and tires and the power rack-and-pinion steering deliver a nice feel of the road, and quick reactions when necessary. At high speeds through mountain passes, the ML550 leans over a little, takes a set, and then grabs the ground and turns the corners.

Critics have complained with some justification about the electronically actuated and modulated Sensotronic brake system that Mercedes-Benz has been feeding gradually into all of its models over the last few years. We're getting used to them, and they'll stop the M-Class right now without much pedal effort. They've also improved considerably since they were first introduced. Yet they still don't have the smooth, linear feel of the best mechanically actuated brake systems. Nice smooth stops can be tricky without practice. We prefer the brakes in the Mercedes cars that do not have Sensotronic.

Off road, the ML550 and ML63 AMG offer the advantages of two significant features: a Downhill Speed Regulation software control that maintains the 4-12 mph downhill pace you set through the wheel controls, and a Start-Off Assist that keeps the vehicle from drifting backward in Drive or forward in Reverse on steep hills. Very handy, indeed, and easy to learn to use.

All M-Class models are rated at a significant 7200-pound towing capacity with the optional Class III towing package, but note the tongue weight rating may be the limiting factor. We have little doubt that three of the four, including the new M320 diesel, would be up for the job (the diesel generates as much torque as a large displacement V8). We'd have some doubts about the gasoline V6 in the ML350, however. If towing a substantial load were a regular part of our routine, we'd look at the ML320 CDI or the ML550.

We've driven the ML63 AMG and it certainly has appeal: a people hauler to do battle with Porsche 911s in the stoplight derby or blast through big sweeping curves much faster than decorum or good sense might suggest. The ML63 is fast, dashing from 0 to 60 mph in less than five seconds, and, with the torque of a large V8 and the high-revving character of a sports car engine, it can be fun to drive. But it's also big and heavy, so don't think sports car. Through the twisties where we sampled it, changing directions quickly and often, the ML63 AMG is not exactly fun. Its massive tires mean plenty of grip. The steering is fairly quick for such a hefty machine, but also quite light, and the package conspires to feel twitchy, almost unsettling.