5th and 6th gen
A week ago I got the 8gb 6th gen and liked it, just not enough to keep. I returned it on Saturday and ordered a refurbished 16gb 5th Gen Nano from Apple online since the only used ones I could find couldn't match the Apple warranty or price. It will only be my 2nd iPod, so I'm a newbie to Apple, but not mp3 players. My first being a 32gb 3rd gen Touch. My new Nano is currently in Hong Kong on its way to the States.
I did not have any problems at all with the 6th gen. It immediately picked up radio with a very strong signal and synced without too many problems. The only ones I had were because of a new computer and the Nano was only recognizing newly ripped or downloaded songs. I had none of the battery problems that other people seem to have had. The size was a plus as well as a minus...while it is was great for working out in shuffle mode you still have to access the touchscreen to change to a different cd, podcast or audiobook. The album art filling up the entire screen was nice but had not a key selling point. In the long run, I needed the larger 16gb size and wanted the classic look of the click wheel which does make it easier to change songs without looking at the screen.
I am looking forward to getting the 5th gen Nano, but for what it is, the 6th gen is quite nice (if not overpriced) if the shuffle format is what you are looking for.