I special ordered my 550i with exactly the options I wanted and it was worth the 2 month wait. This car is an absolute blast to drive. I love the throaty sound of the V8 and the smoothness of the engine. I test drove the 535i also and even though the 0-60 performance is close (according to the specs) the 550i feels much more powerful because of its substantial mid range torque. My car has the sports package so it is a little stiffer than my old car, a 1999 528i (non-sport), but the 550i is not too stiff at all and corners extremely well. The iDrive was not hard to learn and in fact I like it. My coworkers are fascinated by the iDrive and other electronics gadgets. NICE car BMW!
Park distance control (great for garage parking), iPod interface, heads up display, HD radio, sports package (the way it looks and handles), navigation and voice commands.
More console space, and improved cup holders! Also, more exterior color choices.