I always use Castrol Edge 5W30, because that is the best they have on stock at the Chinese BMW dealerships. From what I have read it is a great oil, but it is of course hard to know for sure. I feel fairly certain that it is over-priced in the same way that a Gucci bag is over-priced, but I just don't feel comfortable selecting anything cheaper.
I am also afraid that if I buy anything from anywhere else, it will be counterfeits. Probably not as big a problem in the US though...
Here I do not even fully trust BMW. I always ask them to hand over the empty bottles after an oil change. Not that I know how to identify counterfeits just from looking at the bottles, but at least I can check if they actually used Edge and 5W30, and not whatever was first on the shelf.
Little known facts about BMW: 1) The N54 is still in production and used in the current Z4 35i and 35is. 2) In a Z4 you can get sport+ steering combined with normal suspension etc by selecting DSC=off. (Update: DSC=off also offers sports steering in the M135i) 3) All Z4s with 19" wheels are still only offered with the worst performing OEM tires ever sold with a DCT equipped BMW (Potenza RE 050 RFT).