I had a 6600 for 8 years, and decided to also buy the 8900 mainly for the extra harp space. I just love it. It is quiet, beautiful stitch quality, easy to use. It handles FMQ a treat. I have it set in the Janome table (it fits the table sold for the Horizon) - and after adjustment to the angle of knee lift, found it easy to use. I love the auto tension, ease of stitch selection, easy change of needle plates and Acu feet. The needle position adjustment is very fine and progresses in smaller increments than the 6600, making for exact accuracy. Only problem I had was mine will not sew well with Schmetz needles which I use in the 6600, it was skipping alot But it is fine with Organ, and Superior, which are made by Organ, which are both very slightly longer than Schmetz. At first I used a Janome purple tip 90/14 for FMQ (mostly Isacord 40). I've just (heavily) quilted a large quilt with Aurifil 50/2 using a 70/10 titanium with no problems at all.
I use both machines, but spend more time on the 8900.