My 2007 (launch edition) X5 3.0si was a nightmare. So bad, that BMW bought it back from me (and even graciously paid me for the time I drove it...about 4-5 months).

My 2008 X5 4.8i was for sure better, but not what I would call a good car. From week one, I was always afraid to start it up in the morning...not knowing if it would start, and if it did...what warning lights and messages I would have.

I had battery issues (known problem)
Poor HVAC performance
Rough and jerky transmission (multiple software upgrades did not help)
Randon radio presets "un-setting" themselves, as did the date and time randomly erase or set the wrong time
Frequent iDrive screen freezing. Would have to shut the car off, and wait to re-boot.
VERY poor MPG, even with gentle driving..abou8 8 MPG, 10 if I was gentle

We had a 2004 4.8is, and it was SO, SO awesome. Would I take a new, last generation 4.8is (E53) over a new E70?? In a heartbeat!

Be careful, and JUST MAKE SURE you find an authorized BMW center that will allow you to pull up and PERSONALLY view the service history on the 2008 4.8i in question.

See how many time the car needed help, even though it only has about 5000 miles.

BMW can buy a car back due to problems, and not have to legally disclose it, if the car was not "lemon lawed". Car makers HATE lemon law buybacks, since by law, they have to disclose that on the window sticker, when trying to re-sell. A voluntary buyback, allows the car maker to please the current owner, and allows them to sell it without saying so.
Good Luck!