BMWs and X5s specifically may have many frequent issues: door locks, hatches, CV boot, water pump, etc. But there are 2 things that are bulletproof on BMWs: engine and steering. In fact, it is the German-engineered engines and the steering that makes BMWs what they are: world-beaters. As X Foomph pointed out, there is no reason why a 3.0 engine, or ANY post-1990 BMW engine should not last for 600,000 miles or more. The rest of the vehicle (trim, radio, transmission) may fall apart long before then, but the engine should keep on ticking. Assuming of course that you didn't brutalize the hell out of it by changing the oil only once every 60,000 miles or something retarded like that.