I have the avic d3 and its a great unit, both for sat nav and ipod with ipod video, I currently have a 5g connected to it but soon to upgrade to a 160gb classic.
I looked at various reviews of other systems and even checked them out in car audio places but this one had a great ipod interface. you can browse albums/playlists etc, and the only one of its kind that I have seen that enables you to browse right down to an artist, then the album, play the album and then click "backup" the same as you would on your ipod to go back up one step to view all the albums by that artist again.
I hated other units that took you right back to the top menu again.
the only downside of this unit is the fact it doesnt have speed camera databases on it although for the price its a good buy compared to its big brother that is about £500 more with a built in HDD and speed camera facilities.