To me, it is all about how BMW implements the TT engine. For those who say today that the e92 will be their last M3, I think that is a little premature. Granted, BMW could have an engine that is like the 335i and take something away from the car. Or, BMW could have an engine that performs like the GTR, 911 TT, etc. They could have a high redline for a TT, they could eliminate noticeable lag, there are lots of things they can do to get it "right."
Everyone freaked out when the V8 was announced as it was against BMW tradition in the M. Everyone is freaking out now too. Maybe it will disappoint but maybe it will be awesome. It is WAY, WAY too early to judge just because the word turbo is used.
My bet, and I've said it before, is that a huge portion of people who say they won't get the next M3 because it is a turbo will end up buying one. Why do I think this? I believe (maybe naively) that BMW M will get it right. The engine will be 450+hp, 400 ft. lbs torque, rev to 7,500 RPM and have no lag. The next M3 will beat the snot out of the E92 in all measureable ways and then the converts will come