As one of those former 550 and now M5 owners (who edwardtt10022 might be referring to), I will say that for me, the 550 was a better car. My commute is 0-30-0 or if I take the other way, 0-40-0 and here in the Chicago area, the 550 was just better. Mostly for the torque - the M5 really needs to be wound out in order to get the power going. Getting 9-10 mpg does not help either (the 550 got 13.5).
I still don't regret it, but I will admit, the new 550 is on my M5 replacement list (as is the 535, X5 35i, 335xi).
2012 X6 50i, Vermilion/Sand Beige Perf, CWP, Tech, Sport, Prem Sound, Active Steering, 3 Rear Seat, Apps, Spare. Gone...08 M5 - Sapphire Black/Black - Loaded sans comfort close and high beam assist.