I went from a 550I to A M3 as well. Didn't need a 4 door anymore since I had a SUV and I agree its been a good change. When I went from my Dinan stage 3 540I to my 550I I wasn't happy now going to the M3 I am happy again.
The Verdict: Every other manufacturer should give up on building their own cars and just make M3s instead. -Car and Driver June 2008