Well, I disagree with the majority here, I use BMW dealers for my car.
I had my rear brakes done by a non -BMW mechanic, $900
I had the next car's done by BMW, $830 Yes, the discs had to be replaced in both instances.
I also have access to a free courtesy car, my car is always cleaned, washer topped up, car is always ready on time, and if it isn't, well, I have another, new BMW to drive.
I have found BMW service centres to be extremely courteous, efficient, and not at all overpriced.
I get a nice cappuccino, get to look at beautiful new cars, chat to the dealers about upcoming events (have wangled invites to car launches, and a half-price session at Phillip island in an M3).
Finally, having your book stamped with only BMW Centre stamps IS something 2nd hand buyers look for when deciding between cars. I know I did. Sure, YOU may trust your mechanic, but I don't know diddly squat about him, and I trust BMW to not use non-genuine parts.