I have a 2011 X5 3.5d with roughly 66,000 miles. I am the only owner. The engine was performed great for us; however, since 60,000 miles, we have had some pollution control problems. A check engine light in April resulted in a replaced glow plug and a return two weeks later as the original diagnosis proved to be wrong and it was the glow plug control unit tossing out faults. A few weeeks ago, the electronic throttle body failed. I cannot attribute the throttle body issue to the diesel engine, however, as every BWM I have owned since 2000 (3 sedans in addition to X5) were gasoline-powered and had the same problem -- only at below 20,000 miles.
We have been very pleased with the car up until the first check-engine light incident though the extended warranty resulted in a $50 charge for the glow plug and zero for the control unit. The throttle body was covered under a special 70K warranty (which tells me BMW knows about the throttle body problem. Only time will tell if those will be our only problem.
The engine is quick starting, quiets down at cruise and gets over 25 mpg cruising 78 mph on the highways between Pennsylania and Maryland (traffic flow norm) with plenty of torque to spare. When the fuel-additive warning sounds, you get an ominous message the engine will not start in 1000 miles unless you replace it. I have done it twice although the dealer usually tops it off during normal servicing.
BMW has done a lot to improve their dealer service quality since our first 2002 in 1974; however, not all are fully trustworthy I recommend some research in that department as well, if you plan to use the dealer service department. Having lived in NC, Cincinnati, Fairfield County, CT, Chicago 'burbs and Philadelphia, I have seen the best and worst--although they were all head and shoulders above my experiences with Audi.