I averaged about 19 mpg the day I brought my new C63 home from Torrance, CA to Vegas. Not bad for a vehicle that hasn't been broken in yet.
One thing that disappointed me is that AMG uses a "log" style exhaust manifold which is nowhere as efficient as the "shorty" headers you M guys have. On the flipside, there are LT and shorty headers available to us that adds about 25-30 crank HP.
I like the they way the M3 sounds higher up on the RPM range. Riyatch and I did a roll to 70 from a stoplight and when I backed off, I heard the high pitch revs climb as he sped away.