Nav is definitely nice to have. I wouldn't have bought my 6800DVD if I didn't already own the add-on nav. I just got my 6800 yesterday, and the 6-DVD changer is very cool, although I'm not sure I would ever use it since I don't ever really sit in my car unless I'm driving. I really like the visualizations, although again, it's another feature that's really cool to see for the first time, but I'll probably just leave it on the big clock (and it is a really big clock, very nice). The iPod really isn't bad. It seems fast and responsive to me, much better than what I'm used to. It keeps up with all my button presses. The screen updates are slow so if I press page-down 10-times, it really does page down 10-times but it takes a second or two for the list contents to display. The "Now Playing" screen displays everything you'd see on the iPod screen - all titles and even a cool bargraph. Overall, very nice IMO. The only thing I'm disappointed with is it lacks the wired remote input. I had no idea it lacked this, I may not have bought it had I known. I was just always under the impression that all pioneer x600-series and newer had the wired remote input so I was surprised to find it missing. Oh well, who needs steering wheel controls. 
Anyway, if you want a 6800, let me know. I can ask my source if he can still get any. He got me a brand new one for $200 less than used ones go for on eBay.