For what it is worth, and I am not a gear head to the level many on here are (That's a compliment!). I used the Turner ECU tune on my M5 and I like the improvement. Redline increase, top speed limiter removed, fuel maps, shift points all subtle improvements that are noticeable. Oh, and suggested horsepower increase to 25HP and 20 FT.Lb. Not massive, but noticeable.
No issues, not codes at all. I am really happy with the results. At the time I had to send in my ECU but they have a ship kit now. Just a no hassle option.