I have had the 6500, 6600 and now the 7700. I've loved them all. I traded in my 6500 to get the 7700 and kept the 6600 for about a year just to make sure I didn't want to go back to it. I didn't miss it at all so I ended up selling it.
For me the main advantages of the 7700 are the lighting, the automatic converter plate, the throat space and especially the out of the box perfect FMQing. I have not had to touch tension since I brought it home. I also love the free arm for making bags. The bobbin winder doesn't bother me as I always stopped what I was doing when winding a bobbin.
I'm pretty sure I saw that they are running really good deals on the 7700 right now, so that might be a factor too.
Either way, enjoy your Janome!