I have a Janome mc6000p which I got last Friday - so not even a week ago. I read, read, read everything on here before I bought it and joined the yahoo group, too. That's a great place - search yahoo groups janome 6600 and you'll have to ask to join the group. They have files there where questions have been answered. I did have to adjust the tension at first. (My husband did it so I'm not sure which way he went.) The manual has a good picture of correct tension with advice on how to fix it - that's what my husband used. I have ordered a bobbin made just for free motion quiltiong - it has a blue dot on it instead of the red one that came with the machine. I got it off Amazon for around $25. I am also getting the Magic Genies mentioned here.
I have no trouble piecing, but when quilting was having some issues. It gets better each time I sew. In my case it's a matter of getting used to the necessary speed to get a good stitch - without birds nests and loops on the back.
Hope you get some good answers soon!