I wasn't aware until this weekend that the X5 has the new ISOFIX child restraint anchors in the rear seat. This will be a Federal law starting 9/1/02, and all X5's apparently have them. They are secured by plastic covers in the crease of the rear seat and I believe there are three of them. The rear-facing child seat is SO MUCH more tight than using the seat belt, plus the install take much less time. The only catch is that you have to buy an ISOFIX belt, and the one I found was for my Century seat (about $25 at ToysRUs).

You can also use this for forward-facing seats as well, just make sure you also secure with a tether to the anchor points behind the rear seat...