I have 2010 X5 35d, picked up Feb 2010 and it's got 97k miles on it. I had 2002 Jetta Tdi and wife still has 2004 Passat Tdi wagon. I drive min. 110 miles / day, it's definitely a fun and safe car to drive. After 3 years and almost 100k, I am still looking forward to drive it every day. If you drive a lot like I do, it makes long trips very enjoyable.
Surprisingly, the maintenance cost is very reasonable. I purchased the maintenance service up to 100k, so far the only out of the pocket cost are the tires and two out of warranty repairs about $400 (tag light bulb and extra cabin filters change out). Front tires last about 45k-50k miles, rear tires last about 30k. Brakes about 45k-50k. I did have two emission related issues outside of the warranty, both times are covered by the BMWNA as good will.
Dealer experience is excellent. Maybe has to do with my dealer and SA. They do spoil you and cater to you. If my service takes longer than 2 -3 hrs, they will provide free loaner cars if you request one.
If you are looking to purchased a used one, I will definitely consider CPO and purchase the extended maintenance program. The extended maintenance program may be a little more compare to independent mechanic for oil change and service, but the convenience factor is definitely worth it. Also, if you compare the maintenance program offer by VW, Audi, & MB, it'a by far the most inclusive and extensive.
Also, not sure if you can purchase tire/wheel insurance on CPO. If you can, I will recommend it as well. My X5 comes with run flat, so if you get a nail, you will have to replace it. The tires cost around $275-$400 /tire drive out, so it pay for itself with 2 flats. My already paid for itself.
As far as post 100k maintenance cost I can't tell you. It maybe more than vw. If I still have the car in few years, I will report back my experience with post 100k miles.
If you don't put a lot of miles on your cars, I will find a nice low mile CPO and purchase the extended service program. All CPO extend the warranty to 5 years and 100k (with $100 deductible after factory 50k warranty)
you will not regret the purchase.