I've had a 2005 X5 4.4i for just over a year now and love it. I did purchase an extended warranty as things can get spendy in a hurry on these. The only issue I've had so far is a torn CV boot, which is a common maintenance item. They are typical BMW in that they're very over-engineered and complex, but the payoff is spectacular handling and comfort.
BTW, the exhaust note sounds so good, I had to crawl under the car to see if someone had added a performance exhaust at some point. It's completely stock and I still get a silly grin on my face every time I mash the loud pedal. I shudder to think what a 4.8is would do...
