I've had the 6600 for almost 2 years and really love it. My dealer offers a one year trade up, where within a year, you can trade up to another machine and receive full paid price on your trade in. This is what my machine was. The previous owner traded up to the 12000.
The 6600 does everything I've ever asked it to do. I've FMQ'd several twin and full sized quilts on it. It's 9" harp is plenty big for queen and kings, I've heard. But I haven't had the occasion to try it. My friend owns a Gammill with the computerized stitching, so when i have a queen to do, I do it on her machine. It's a nice treat, and I'm not sure I want to wrestle that much fabric. Just binding a quilt that big is a workout.