After investigating and test driving many V8 X5's and Cayenne's (they were the only 2 models I had interest in) over the last 3 months, I pulled the trigger on a 2005 X5 4.4. Both trucks had lots of giddy-up (the Cayenne obviously faster) and handled very German-like. The decision finally came down to exterior looks, maintenance costs and MPG as the rest was a wash for me.
I filled up the tank last week and drove 550 km's before the light went on. I figure I've got at least 50 km's left so that would make it 600 km's on a full tank of gas. Granted, I did not push it much in the first week of ownership but it proves that the gas mileage ain't as bad as most claim. In fact, my buddy purchased a 2006 X5 3.0 a couple of months ago and he has never gotten 600 km's on a tank.
Just my opinion, but the V8's make the X5 a much different driver than the 3.0. A much preferred difference for me.