I quilt (up to queen size so far) on a 6600P and don't have any problems. As already said lots of practice, good thread, right needle and get the tension right. Even machines of the same model have their own little "quirks" you get to know.
Depending on which Janome you have, you can get a bobbin holder (with a blue dot) specifically for FMQ, and there is also a set of feet for FMQ.
The Janome 6500/6600 mentioned by Kim above is very good.