I have had an '08 4.8 since Oct and the exhaust note is worth it alone. When you drive the 3.5i and 5.0i back to back--it's a pretty easy justification if the $100/month + extra $ etc is not a deal breaker for you.

The other option for is if you didn't "have" to buy brand new you could pick-up an '10 4.8 that would give the V8, the new iDrive in it and also the 214 style wheels (most people like those over the 5.0i wheels). I bet you could get one in the low-mid $50K range. I know mine stickered for $75K 2 years ago and got it for $48K with 16K miles on it.

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2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film