Handles worse than an M3.
Much faster acceleration than an M3.
Ride quality is about the same.
Interior fit and finish is better.
I wouldn't own one w/o warranty though. I bought a new 2010 M6 last year because I wanted a full warranty plus 2010 is the only year with CIC iDrive.
Sounds brutal with an exhaust. I have the RPI GT exhaust and can't get enough of the sound. My M3 also has a great sounding exhaust but this is on a different level.
My M6 gets 14mpg, my M3 got 16.5mpg.