I've taken my 911 here for a bit over a year (since the factory warranty ran out) and have been pleased with the service. They have been very up front with what I need and what I don't and trust their recommendations. On top of that they charge reasonable rates (or, at least what I think is reasonable). I recently had to have the tiptronic transmission redone (oops) and they did a very good job. I don't know about their Mercedes or BMW service, but I recommend them for Porsche service.