Another vote for the 6600 here, I purchased mine used 6 years ago and love it! $899 is a good price for it used. I see them regularly for that on Craigslist here, and have been tempted it purchase another so my daughters can use it when we sew together.
The 6600 is wonderful for quilting and I have managed to quilt quilts up to 95x95 on it. It's a challenge when they are that big, but it is possible.