I'm two weeks into being a 7700 owner and on the whole am very happy. The most important issue for me was free motion quilting, because I didn't have a machine that would do it satisfactorily. So far I've not altered the tension or any other setting on the 7700, have tried various threads and battings, and have perfect stitches front and back. That alone would sell it to me after all the years of struggling with my old machine. The one problem so far is the one that Nanbue mentioned - a reluctance to go over bulky seam intersections - which may mean that the feed dogs need a slight adjustment, but I want to spend more time trying the dual feed foot first to see if that solves it. I can live with it if I can do the FMQ - I have 2 other machines that will sew over anything!
I was originally planning to get the 6600, as I thought the 7700 was out of my price range, but a good offer last month made it just about possible, and given the larger space for quilting, I decided to go for it.