I've taken my BMW to Mustafa at GT Exotic Cars multiple times... he works out of the Exxon garage located on the South-West corner of Passaic and Allwood Rd. You can always trust him to give solid advice and do a high-quality job on your vehicle. Competitive pricing for top notch work. Highly recommended.
Edit (Aug 2014): I was told by the owner that this shop has changed it's name to "Atlantic European Automotive Service LLC" but same owner / mechanic.