I drive a 2007 4.8i with the 7-seater option. The power is addictive, and for such a large, powerful car, the fuel consumption is high, as expected, but not a killer. Driven gently, especially on longing trips, I get 13 l/100km (don't know what that is in mpg).
Being of the first new model year (2007), there were a few minor niggles that were taken care of under the maintenance plan. Some issues with the a/c, mostly, nothing mechanical at all.
I love the car, and my only complaint is that the ride can be stiff over bad roads, due to it having run-flat tyres. My wife's X3 has a more compliant ride than the X5. OTOH, the first set of tyres lasted 75000 km (47k miles). My advice, avoid the very low profile 20" rims and the sports pack. The 4.8i comes standard with self-levelling rear suspension, so the car can swallow tons of passengers and other load without complaint. The 3-rd row seats are only good for smaller children, maybe up to 10 yrs of age. Although the middle seats can slide forwards, there is limited legroom for the 3-rd row seats. The regular rear passenger space is huge, though. This is a very comfortable car for 5 adults.
It is a lovely towing vehicle as well.
You should also look at the new X3 just released (don't know about the US market). It comes in the 35i version, with the 3 liter twin turbo 6 cylinder. Wicked power with surprisingly large interior.
