I upgraded as soon as the 7700 came out from my 6600. I found the lighting to be better on the 7700, the free arm for me was a must have, and the machine is quieter than the 6600. FMQ is fantastic, never had any hiccups or burps with it. I love the stitch plate converter that is built in vs. monkeying with the plates.
What I miss about the 6600, is the bobbin winder the most, where I could sew and wind at the same time. I made a separate cover for the 7700, to accomodate the thread cone attachment which is not standard.
I also have purchased the 12000 Horizon Embroidery model, and kept the 7700.