I feel your pain. I bought my X5 4.8is last May, and to date the air suspension has been the thorn in my side as well. It's been at the dealer since Tuesday getting the suspension air pump replaced, as well as peeling interior door handles and peeling steering wheel trim replaced. Both front air struts have been replaced as well. I thought maybe my $3700 extended warranty was a bit steep, but since I've had it in for over $5000 in repairs, I guess I'm ahead?!?
I love the vehicle, but one more suspension related issue, and I think it will need to go to a new owner. I'll replace it with another BMW, but one with a "conventional" suspension setup. Hang in there...maybe it will get better....maybe.