I have a 325xi that is a year and a half old and I've already put 57,000 miles on it. At about 36,000 miles, I had a problem with my ignition coils. They were, of course, replaced under warranty. However, other than that, I haven't had any problems to date. I feel it was just a fluke. This is my third BMW. The last one (318ti) I drove 190,000 miles and for the first 150,000 miles, I really didn't have any major problems. It took 4 or 5 rear-end hits during the 7 1/2 years I had it with minimal or no damage. I was impressed. After 150,000 miles, I had to replace the fuel pump, window regulator & motor, and some fans. The radiator as well as all the major components of the car were still good after 190,000 miles when I sold it. There was never a transmission problem.
The way I think of things, cost doesn't always determine reliability. For example, a $40,000 BMW may need maintenance more often than a $30,000 BMW simply because there are more things on it to break. They could be the same car except the more expensive one has all the options and the other one is just a base model.
All of my cars have been manual. I always felt that if there is ever a transmission problem, it would be easier to fix. However, I've never had a problem with it. Whether or not the automatic ones are as reliable as the manual ones I'm not sure.
You can't trust any car to be 100% reliable. Even if is, that doesn't mean that another car of the exact same model and specifications will be. BMW has a good warranty. It may only be 50,000 miles but it covers everything (except tires), even brakes, oil service, inspection, etc. I feel that 50,000 miles is plenty to determine if there was a manufacturing defect or a faulty part.
In any case, BMW is the only car I buy. My good experiences with them significantly outweigh my bad ones.
Maaz