A buddy of mine bought an M6 when they first came out. It was a fun car, I can say from first and secondhand experience that some of those cars have been used and abused. That is one car that people tend to buy that are actually interested in performance.
The V10 is awesome, revs like you wouldn't believe. I didn't care for the SMG gearbox, but I guess it is the best option. It was great when you were beating on it, but around town it felt clumsy, slow to react and not smooth at all. Brakes were great. Car felt kind of wide, but I think it was just the shape of the front of the car combined with the view from the driver seat that made it feel wide.
This particular car used a quart of oil every other tank of gas and the back tires didn't last much longer. And it wasn't a kid who owned it.
For me, the M5 would be more practical than the M6. It seems silly, but the sedan is actually useful.
One thing I will say, I think he owned it for two years and lost about 50K in depreciation. That is a big hit that would make a used one very appealing.