There's an awful lot of speculation in this thread on both sides, just as there is each time Dinan is brought up.
I've heard horror stories, but my personal experience has been great. I happen to have a good relationship with my SA at the BMW dealer at which I bought the car used (in fact she referred me to Dinan, which is a local company for me
), and had a few engine-related repairs done (as well as continued factory maintenance) even with a few Dinan parts and a tune. The one thing I can say with certainty is that it's probably a good idea to check with local BMW dealerships and/or Dinan beforehand.