I have the 6600P and LOVE LOVE LOVE IT! Before this machine I had a Pfaff 2056. After getting the Janome, I couldn't sell the Pfaff fast enough!
Be sure to read the manual and take a couple of days to just play around with it.
Short of using postal twine, I've not found a thread yet that I cannot use.
I really like using serger thread for garment construction and quilt piecing and the machine runs it like a champ.