I've owned both... and the IS-F will FLAT OUT own the M3 in every performance category... drivers being equal. And I'm more than happy to line up with any 'Local' M3's... in AZ. My M3 had RPI Off Road, Riss Racing Axle Back, Tune, Scoops, Filter, Crank Pulley... about every bolt-on... my IS-F has a Cat Back and a drop in filter.. traps .3 quicker, and 3 full Mph more than my M3. I school M3's all day long when given the chance.
It's not to say the M3 isn't an amazing car... it is... but the IS-F has more Torque, has significantly better brakes 'Real 6 piston Brembos'... and the nail in the coffin.. gets just over 22 MPG in day to day city traffic. May not mean much to some... but as we approach $4/gal it will!
Lastly... the 6 speed manual in the M3 is crap... only way I'll ever buy another one... would be DCT... and that's from a guy who actually road races his cars... the Manual is so notchy, grinds if you try to shift with any 'swiftness' on the 1 - 2 shift... was very disappointing for me.
I still respect M3's... but I checked the times page here... and I have like 3 seconds on the fastest 'modded' M3 against my 'Stock' IS-f at Cal Speedway (Auto Club).
Enjoy the cars how you like... there is no loser with either car, including the C63... just different cups of tea. The bashing that goes one from guys that have never owned the other makes, is amazing to me.
2010 ZR1 3ZR Wht/Blk 10.7 @ 132
2011 C63 AMG P31 Car Blk/Blk 11.8 @ 117
2010 Nissan GT-R 10.8 @ 129 (Sold)
2008 Lexus IS-F (Sold) 12.5 @ 113 / 2008 Shelby GT500 (Sold) 11.3 @ 126
2008 e90 M3 6MT 12.8 @ 111 (Sold) / 2006 e60 M5 12.4 @ 114 (Sold)