I had been drooling over the 6600 for quite some time and finally had an opportunity to by an "experienced" machine for a really good price after the 7700 came out. of course once I saw the 7700 I wanted that ... but settled for the 6600.
Yes, the integrated walking foot ("accufeed") is well worth the $$. I love it for strip piecing. FYI - I set my stitch length to 5.9 for regular cotton fabrics and 5.7 for batiks for my "scant" 1/4" seam and I use the accufeed foot with the 1/4" guide.