it's all about the engine. the 550 is just BOOM. it sucks gas, but then again if you're looking to conserve gas this isn't the car for you.
if you want power to move, pass and flat out haul a$$, the 550's your car. i've found it handy on the interstates. it's nice having that extra power--and the car with a sports package can handle it. i would want 19" wheels + dynamic damper.
i have xdrive for my really crappy weather. i have a set of -1 rft Dunlops (OE BMW PN) 18" wheels which makes the car go through anything.
i could live with a 535 again, but the 550 is about as close to a perfect all-around car. you can stiffen it up and it handles great, or you can soften it up enough to transport someone back from a hospital that's just had major surgery in great comfort.