I had 2003 3.0i over 80.000 miles - no problems what so ever. Now got 2005 4.4 some issues with tranny jerking. Dealer is working on to get it fixed. My experinece is that 5 speed auto is pretty reliable and very smooth. 2004 they put new 6 speed auto in and that seems to have some issues. Go for 4.4i it sure has more torque you'll need when towing !

If you can have extened warranty get it.

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X5 4.4i 2005
sport, premium & cold package
328i 2008
premium package