Hi - Thought I'd throw my two cents in - I've had both of these machines. I started with a 6600, then traded up to the Horizon 7700. I didn't like it as well as the 6600, so I sold it and bought another 6600. The only thing I liked better on the 7700 was the larger throat space. Several things that drove me crazy - The way the spool of thread sits on the 7700 and the way it winds through the machine (on the top, under the lid), and also to wind bobbins. I found that cumbersome. Also, the (optional) thread stand for larger spools the 7700 is stationary, it does not fold down like it does on the 6600, which makes impossible to keep a cover on the machine when not in use. Probably the main thing I disliked about the 7700 was the bobbin winder. The 6600 has a separate bobbin winder - You load it up and push a button, really easy! The 7700 needs you to use the foot pedal or unplug the foot pedal if you want to use the on/off button to wind the bobbin. I found this cumbersome also. In general, the 6600 feels more sturdy to me, it feels more robust and solid when it sews. Also, as others have stated, getting the 1/4" seam on the 7700 is problematic, and many on the Yahoo Janome 7700 list speak of the difficulty of going over thick seams.
Solomae