I have owned 4 BMWs and several other cars. The 2006 M5 is the most fun, practical car on the market. The SMG transmission is a blast - very versatile, fun to shift, useful but not great in auto mode. I get comments all the time about what a great car it is. And I love the iDrive. Yes, I would do some things differently, but overall it is very easy to use. Only complaint about the car is that I would like to see the dashboard more driver oriented like the E39 was.
SMG, engine/powertrain is amazing, iDrive, looks. Did I say SMG?