I used to have a Cayenne Turbo S a few years ago. It was a lot of fun but all that fun was constantly ruined by small, yet very annoying defects. It was under warranty, so everything got fixed by the dealer for free, but it did feel to me that I was driving around the dealership. First the back seats got stuck in the fold-down position because the seatbelt got stuck. Then the Nav / Radio system would consistently reboot itself at random, sometimes every 20 minutes (took 3 trips to the dealer for them to figure out what was wrong with it - turned out some component in the fiber optic network was failing causing the entire thing to reboot). Then there was water in the front passenger foot well - because of some dirt in the AC piping, and they literally had to take half of the car apart to clean that... It was powerful, but squeaky at times, and it just felt there was always something I wished they did different when they designed the car.
Now I have a 5.0i E70 and couldn't be happier. It's not even an M, but with sport package among other things - and it feels a lot better than the Cayenne. I'm sticking to the BMW, and probably will never buy another Porsche again.