My wife traded her A4 on an A6. I liked driving hers so well, I traded in my IS 250 for one too!
Fuel economy, range on a tank of gas, MMI, satellite radio (don't know just how much I would like it), CVT transmission - puts conventional automatics to shame, paddle shifters, quality of interior materials - especially the wood trim, seat comfort, Bose stereo, full sized spare on alloy wheel.
No iPod integration or AUX input, CD changer doesn't recognize MP3s (this is fixed on 2007s), positioning of A/C vents chills hands, 18" wheel design a little bland, warning triangle no longer standard equipment
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