I went from the 6600 to the 7700 to the 8900. I loved the 6600, regretted buying the 7700, and LOVE the 8900. I love the basics - how it sounds, how it reacts, how much space and light I have, how easy it is to use. The major improvement over the 7700 that I appreciate are:
1. You can change the needle plate with just a press of a button instead of screws.
2. Snap on Acufeed feet - standard, 1/4 inch, stitch in the ditch
3. 9mm stitch width
4. More alphabet options.
I do piecing, free motion quilting, stitch in the ditch, and machine applique. I can make the changes on this machine quickly to go from one to the other. I have had no problems with the tension for free motion quilting - even with metallics and monofilaments. The stitches I use for applique lay flat and tight. I have used the fine Bottom Line threads to the coarser/heavier quilting threads and the automatic tension seems to adapt.
Hope this helps.