I bought a 6600P specifically to be able to do fmq and I haven't been disappointed. It takes practice as it will with any machine. The optional bobbin case with the blue dot is specially for fmq and takes care of any tension problems - although I sometimes use the supplied bobbin with no problem - depends on which threads I'm using. The large bed makes manoeuvring a large quilt easy. I have the table for this model which improves ease of use even more.
The accufeed system is excellent - never a pucker or trouble with quilting layers.
I'm a fan!