I only have the Horizon and I really like the features others have mentioned. I upgraded from a cheap Pfaff with almost no bells and whistles (but sewed fine) with a small harp/throat space. Having all that room makes a WORLD of difference. I also like the lights. I like the even feed, thread cutter, needle threader, the easy way to put the feed dogs down and just how easy it is to actually quilt on it. I have only actually FMQed about 4 quilts, but I think that I am going to learn to love it using this machine. I've been using it to sew for almost 2 years now and have some some stitch in the ditch--but the FMQ seems so much easier.
So, while it seems expensive to jump up to such a machine (from a $200 Pfaff), I'm so glad that I did. The Horizon is a quality machine--you can feel it--it is sturdy, runs steadily and seems to be a workhorse. I'm very grateful for mine.
And I'll have to admit before I bought this one, I'd never heard of Janome. (Blush, blush). Pfaff, Bernina, Singer, Kenmore, New Home, etc...but not Janome. But when I wanted to buy a 'higher end' machine here in Poland, my choices were very limited. For me it was good as it cut down on the shopping time. I compared by using reviews on the internet and ordered...
But as far as I know, people are very happy with their 6600s. I paid 5000 zl in 2010, which was about $1900. I think compared to what most of you are having to pay, that was a good deal. I read that just now some are paying that for the Horizon. I don't know what a new 6600 would cost here.