I used to own a 760Li, X5 4.8is and an M5 (all MY2006 - yes, those were better days..
They were all very reliable. But I have to say that I wouldn't go back to BMW or a different European brand at this tome because many U.S. manufacturers like Jeep or Ford were able to close or minimize the gap when it comes to quality and technology.
Spending twice the money on these cars (when comparing an X5 to an Overland, for example) is NOT an option anymore - even if I could afford it again some day. The gap between an 06 Overland and an 06 X5 was more substantial...but not any more.