I had an 03 X5 3.0 with the Dinan AT and Performance software and 20" Type 87 wheels and it was okay for day to day but I really hated things like merging and passing cause the engine is just plan slow...for the size of the car and the speed of highway traffic in my opinion. Yeah it gets to 80 mph but dang if it doesn't take too long to get there. Alot of times I can remember watchin the 18 wheelers bearing down on me and looking at the speedometer and pleading for it to go a little bit faster!
So I got the car in Aug 2006 and traded it in Feb 2007 for an 03 4.4i and haven't looked back since! The v8 is smooth, gas mileage is basically the same give or take 1-2 mpg's and the extra torque is very nice! They even upgraded it to the Dinan Performance AT and Upgraded engine software too on my 4.4. so other than the extra $$ spent refiancing a newer car...it was an overall pleasant and worthy experience!
I'd do it again most definately without thinking!
PS...also looked at the new 2007 X5 3.0 vs 2007 X5 4.8 but IMHO wasn't worth the cost they're askin for those right now.