I have the 6600 and I love it. I looked at upgrading it not too long ago but decided there wasn't enough "bang" for the buck to upgrade. If I didn't already have a really good machine I'd have bought it though. Snap-on Accufeed feet would be WONDERFUL, ditto for snap on/off needle plate.
I can't use the knee lift on mine, either. It's just not a natural motion for me. It sits in a drawer.