I have both the 50i and the M and both are trouble free fast and powerful trucks...I had the N63 campaign work done back in February and BMW will cover any part replaced up to 2 years. Since the work the 50i has ran perfect but it still drinks oil like it drinks gas...I recommend getting the M, it's a lot more fun to drive and with a few mods you have yourself a BEAST on steroids!
There's nothing like having 2 gas guzzling slobs