I had a 2003 3.0, and really enjoyed it. It drove great for an SUV, and had enough power, but eventually I got bored with it. It was too heavy, had a higher centre of gravity, and just wasn't as engaging. I bought a Z4 3.0 with sport, SMG, etc, and it was fun for weekends, but I didn't use it enough, it spent too much time in the garage. So, I went to a 535 with a 6MT for a daily driver, and I enjoy it more. Sold the X5 and the Z4. Still needed a utility type vehicle, so we got my wife an X3 3.0si, 6MT. I enjoy driving that as much as I did my X5.

IMO, if you enjoy driving the 335 and appreciate what it offers, you have to be ready for a big change with an X5. It is a great vehicle, but it is nowhere near as tossable. As was said above, it handles great for a truck, but you never forget that it is a truck (OK, SUV). I would get your wife the vehicle she wants (X5 3.0) and figure out what you want in 2010.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue