I have an e90 m3 and we use the Chicco brand car seat. It is rear facing and the best part about it, is you drop the seat in and it almost always finds the locked position. I didn't like the Graco or brittax seats for the base lock mechanism only. It is also a lower price brand.
09 Space Gray E90 M3 DCT: BPM (Stage 1, DCT and Servotronic) tune, Gintani Sport Exhaust, Dinan Intake