I've been driving X5s since the first model and leased an early production 2012 over the summer. I still find it amazing that I can place an order for the factory to custom build an X5 that arrived at my dealer in 3.5 weeks. It now has over 11,000 miles on it and it has been completely trouble free (as have my other X5s with the exception of the first model over a decade ago.) Even after test driving other mid-sized SUVs, I still go back to the X5. Aside from the technology, I find the driving position and the multi-contour seats more comfortable than any other vehicle. I've never had trouble operating the iDrive system and frankly am tired of people complaining about it because they couldn't figure it out in a 10 minute test drive. No manufacturer can completely idiot-proof a system.
Several options deserve consideration: contour seats, top-down camera that shows you the sides, front, and rear of the car, and heads-up display (once you have this, you won't want to drive a car without it). The new engines and the 8 speed transmission are noticeably smoother than the last generation 6 speed.
If you are leasing, find a dealer who will show you the lease residual chart for the different X5 models. There are huge differences in residuals between a 3.5i, 5.0i, and 3.5d that will greatly affect the lease payment. For example, the 3.5d had a residual of 51% and the 5.0 had a residual of 60% for a 36 month lease with 15k miles. That 9% difference means that the 5.0 was the same lease payment even though the 5.0 is more expensive and the diesel had an additional $2,500 eco-credit for 2012.
I'm hoping to hold onto this X5 until the 3rd generation is released in two years.