Since I bought my '05 last summer, it has been to the dealer every 3 months or so (have extended warranty). Mostly just nagging items. Only real driveability issues have been a faulty telephone module killing my battery and a front driver-side air strut leak. Not really things you can check for during prebuy, unless battery is dead and you have the selflevel susp annunciator. Since these are typically the most optioned out X5s, they have more fancy gadgetry to randomly fail 5 years down the road. Don't get me wrong, I love her. Currently have about 54,500 mi. Would definitely recommend some sort of warranty as this will not be the most reliable BMW you will ever have.

Scott