If you go roof route check the high roller carrier from Yakima to use w your BMW load bars. Best / easiest I have seen.
If you've got shoulder or back issues and roof isn't a viable option you could give this a shot. Looks easy to load but only does one bike, is stupid expensive and requires some sort of adhesive?!?!
http://www.shopbmwusa.com/PRODUCT/38...ICYCLE-CARRIER
I'd probably hold out for a tow hitch kit and go that route instead.