James, just read your review (I am always late to the party) and you pretty much nailed it. We've put 10,000 km on our 07 MS3 and, like yourself, have driven the regular M3 in the past. I was surprised at how improved the performance thresholds are for the MS3 over the standard variant yet it still remains civilized when you want it to. Best of both worlds.
Engine - moments of lag under hard acceleration, but great mid range torque, every where, every gear. A+
Drivetrain - clutch and tranny have a solid "don't rush me" bulletproof feeling. Guess they have to be...we'll see.
Chassis - stiff body and forgiving chassis mean great tracking when pushed but still compliant over the rough stuff.
Exhaust - low grumble, NOT ricer sounding, does not exhaust you when cruising at hwy speeds.
Cabin - very flexible, lots of headroom, grippy seats that are great over long trips, good visibility.
Brakes - supercar good, truly. Very easy to modulate close to the ABS threshold.
Gas Mileage - much better than expected. 11-12s when in speed racer mode, 8.8 avg over highway trips
My wife loves it for commuting and long trips, I love it for the twisties. Great car - great value.