Here's the good, the accufeed system, the 1/4 inch foot, the price, the throat size, the durability, the auto threader, the thread cutter, the button system for stopping,etc . Here's the bad, that crappy screw/bar setup to attach the accufeed feet, NO bobbin thread empty warning, tends to come unthreaded from the uptake level all the time, tends to make nests if you are not careful after using the auto-cutter, and the manual.
I am happy with the 6600 and I use mine nearly every day. I cannot justify double the price for the 7700, even though it is nice because I would not use the extras. Nor can I justify the money for the 12000. Everybody is different.