Just picked up my used 09 335i m sport, and this was the first thing I tried cause it was free and super easy to do.
Yes like some others said, it was like 10% louder than stock at idle and low rpms, it was cool at first...
but the loss in low rpm torque was VERY noticeable for me and felt almost sluggish booting it off at the usual default stop lighted 2nd gear in sport-tronic mode.
The next day I plugged it back in, and the car gives me smiles again!! Even in 2nd gear from a stop!