I've just got the 6600 and I LOVE it! Having quilted up til now on a standard machine, the wide throat, speed control, needle up/down, acufeed system are the things that spring to mind as huge improvements on what I had. I'm sure they'll be more, the more I use it. Even putting on a binding is way easier and more accurate. I haven't pieced on it yet - been FMQing a lot and the quality of my work has improved immensely! It was pretty straightforward to get to grips with as well - it has lots of features but to me seemed sensible and logical as to how everything worked - although I had some demos and lots of advice from Katrine who posted above so had a bit of a headstart! :-D