E46 M cars needed routine and expensive valve jobs and the SMG transmissions were not held in the warmest regards.
E9x M cars needed rod bearings and although the engine was a gem, it was highstrung and needed more attention then your typical car.
I'm about a year out from full blown car shopping, but the more I look around the more it looks like a lightly used M, RS, or P car is going to be my play rather than getting something new. The F chassis M cars are the first to have a turbo charged 6, I would hope that means the S55 engine didn't have to be as highstrung as the S65 and S54 predecessors before it and thus may be a little easier long term. However, these cars have also become vastly more complex and electronic, whether that be the idrive system or the water pump.
I'm no virgin when it comes to BMW maintenance having owned my e46 330ci from 75K miles to 154K miles, and an E90 335i (N55) for a year and 22K miles. TCL lend me your knowledge, anecdotal or personal experience (I know we have at least a few owners in here). Obviously these cars are still newer, but any longer term info is appreciated. Weak points, strengths, give it to me.