I have the Janome 4120 and the 7700 -- I used to have the 6500. The 4120 is great for lighter weight fabric and has a lot of great features. The 7700 is a free arm with accufeed (which the 6600 also has-- this is a built in "walking foot"), but it is more money. I take the 4120 to quilting class and it works great, though it does struggle when a lot of seams intersect. The 7700 would probably eat fabric if I let it, but I always (on both machines) start sewing with a small (at least 1" square) scrap of fabric before I put my good fabric through the machine and I don't have a problem. The 7700 also has the single hole needle plate that is built in and slips out when you need it. It has more stitches (some really cool ones for doing little girls dresses). Here are a couple of places where you can get more Janome specific information:
http://groups.yahoo.com/group/Janome6500/
http://groups.yahoo.com/group/JanomeHorizon7700/