I am on the verge of trading my X5d for an M5. My X came with the 19s too. Over the summer I put fender flares and 20s on. I mentioned to the BMW dealer (that I'm using) during trade-in conversations that I had 20s and he is now currently trying to pry my 20s from me and include them into the trade-in number. So far, his number is pretty good IF I include my 20s. I guess I'm saying that in this experience, the dealer wants the 20s (or included in the deal), it makes it a more desirable auto. He didn't seem to care much about the 19s.