I've been a happy 2010 X5 35D owner since mid-October. I don't know if I will ever going to go back to gas! It just makes so much sense, particularly on a heavy truck. Economical, tons of power on demand at cruising speed with no downshifting necessary, great acceleration (yes, there is a milisecond lag, but its not worse than my gas drive by wire e-class), and satisfaction of knowing you are rational and not wasteful in your luxury car... I am not sharing any secrets here, diesels are as great as they say they are, and I fully experienced it.
Yes, some people had issues with their cars, but I am not convinced that defect rate on diesels is any different than any other high tech car, including other BMW models. My brother's 330 had its share of problems, but he still loves it. I was lucky for not having experienced any issues yet, so put me in the statistical group of those who don' write to the forum because everything is working fine.
But the most important thing to me was that my wife, who complained about every previous car I owned, is absolutely in love with the X5. Its hard to value that.