I've had mine for nearly 4 years now and never had one problem at all.
It stitches beautifully and seems to love all threads.
The Accufeed is wonderful, and if you get the machine it is well worth buying the Accufeed quarter inch (which you normally get the small hole needle plate with as well), STID, and open toe ones too.
I bought mine especially to do my own fmq on and have been able to do that better than I ever thought I could. I also got the convertible fmq set of feet, and the blue dot bobbin.
The machine sits quite high, so it is much easier to have it "sunk" into a custom table. Some make their own - but I got the 6600 metal table when it was on promo and it makes all the difference.
I wouldnt change mine for anything else either!