The GL has a unibody frame — rare among full-size SUVs — and its four-wheel-independent air suspension swaps traditional coil springs for inflatable air bladders. Its ride height can be varied to enhance high-speed stability or offroad ground clearance. An optional adaptive suspension constantly adjusts ride stiffness as conditions require. My test car came equipped with the system, which, combined with its 21-inch wheels and sporty P295/40R21 tires, resisted body roll admirably.

Like many adaptive suspensions, the GL550's has Sport and Comfort modes that yield stiffer or softer response, respectively. There's also an Auto mode that calibrates the settings somewhere between those two. Sport mode resists body roll marginally better than Comfort, but the difference is slight. On the highway, the differences are all but unnoticeable.

The steering wheel's precise turn-in makes curvy roads a pleasure to drive. There's enough power assist at lower speeds for tight parking-lot maneuvers, though at times I found lumpy pockets of resistance while navigating our downtown parking garage. It's a noticeable difference from the steering response at higher speeds, which is smooth and confidence-inspiring.

Alas, the GL550's advanced suspension can't make up for the brittle ride quality of its thin 40-series tires. You'll hear and feel every highway expansion joint, while potholes and speed bumps become a jarring, noisy experience. Most annoying was the abundance of suspension reverberation I felt around Chicago's roughed-up streets: No matter what setting I had it on, major bumps sent palpable motions through the entire chassis for a half-second or more.

On smoother roads — that's you, California — the ride feels more agreeable. Tire noise is still noticeable, but wind noise is minimal and the engine is nearly silent. I can only presume that the GL320 and GL430, with their 18-inch wheels and thicker P265/60R18 tires, would be more comfortable on bumpy roads, but be sure to compare them on your test drive.

At 39.7 feet, the GL's turning circle is competitive with its peers.