I had the 7700 for about 3 years before I upgraded to the 15000. I loved it. The sewing over multiple layers was solved with a new 1/4 foot called the "O2" (the "O" was included with the machine). So if you are testing the machine with multiple layers, make certain you are using this foot.
Like many machines there were reports of some that had issues, but I would say there have been many owners that truly enjoyed this machine (and may still, but I no longer keep up with the Yahoo group).
I agree - if you can test the machine, do!