This is a great car. I moved up from an Acura CL-S, so it was an upgrade on all fronts. The acceleration is silky smooth. For the most part, the engine is quiet but still can let out that great sounding growl when you really jam the pedal. There's no cooler sound than rolling through a short tunnel and hitting the gas just before the mouth It's like a sonic boom! Mine is fully loaded, with the P3 package and just about every option available. I think the P2 package is enough though, and you don't need silly options like the automatic trunk opener/closer.
I would highly recommend getting the 550 over the 350. This stout engine is really worth it. Whether it's from a dead stop or already going 60 on the highway, you always have a ton of power in your back pocket whenever you need it. With its top down, the CLK has few rivals in terms of looks.
I think the navigation system needs some work. My Acura's nav is much more intuitive and useful (and it's 8 years old). It needs to be a touch screen as entering addresses takes forever using a joystick. Sound system needs a little tweaking. I like my Acura's Bose better than the HK.