I have the 6600P. It took us a short while to get used to each other, but once we did all is well.
I love the accufeed food and use it a lot, even for piecing. I have a 1/4 accufeed foot as well as the regular. I do have to adjust my needle position for a proper finished 1/4" seam though. I purchased it as a companion to my Husky 535 so I could keep it set up for piecing and use the Janome strictly for FMQ (I wanted the larger harp). I like the accufeed so much that when I'm piecing ... I still use it. I'm neglecting my Husky!!
In the beginning I had more than usual thread breakage. As I said, this has all but stopped. I will still have some issues at times with a lesser quality thread. Neither my machine or I like Connecting Threads brand "Essential". My machine would at times break only 1 ply of the thread leaving the remainder of that ply to bunch up somewhere in the machine (usually the tension discs!), and I also found the thread incredibly "messy" - fuzz EVERYWHERE after only a 1/2 or so of usage. As a result I use mostly higher quality threads (Aurifil, Superior, Gutterman).
I am a frequent cleaner of machines. At least every bobbin change. Because I'm taking the darn thing apart so often, I wish the face plate was easier to remove. The screw at the back is little hard to get to and I have very tiny hands. This is a minor *complaint*.
I love FMQ on this machine and have even started thread painting on it with great results.
There is not much I don't like about this machine.