I did go with AMC, they were professional, but not my kind of garage. Impersonal and no interaction with the guys working on your car, just some dudes in white shirts and black ties who man the reception desk.
Taking the Z4MC in there just meant there were 15 mechanics all wondering over to gorp at my car, after about 15 mins of discussing who was going to work on it (and who got to ride shotgun in the MOT deccelerometer test), whilst i was stood in the waiting room watching through a large wnidow.. they started work and got the job done relatively promptly. Didn't fill me with confidence though (probably me being irrational, but i'm protective of my car, hopefully understandably), so i'll probably goto an ///M specialist next time where my car is less of a source of excitement.
If i was taking my mates 330ci, i'd of happily left it with them - that's the standard cars they seem to process (saw lots of E-class, 5 series, 3 series etcetc).