I FMQ on my 6600 and it works great. I don't even have the blue dot bobbin case - don't need it. I change my foot, loosen presser foot pressure, and just go for it. I don't usually change tension and I don't bother to drop my feed dogs either. It's SO easy. I swear this is THE most cooperative machine I've ever used. I can still screw it up by going too fast on the curves and getting eyelashing, but if I stick to a reasonable pace everything turns out great. (Well, aside from my wobbly lines and other mistakes, but that's not the machine's fault!)
I looked at upgrading to the 8900 model but after test-driving it, I liked my 6600 better.
The 8900 has some good features and I think it's a more attractive looking machine, but it wasn't enough to make me abandon my beloved 6600.