For future reference, you can go to
http://sewing.patternreview.com/EmbroideryMachinery/ to get reviews on all makes/models of sewing and embroidery machines. Unfortunately, although the 900D is listed, there aren't any reviews on it.
I've owned the Brother PE-150, 8500D, and now have the Innovis 4000D and I've been extremely happy with them all.
You are restricted to a 4x4 hoop with the 900D so you are limited to the size of the designs you can do. And it is a combo sewing/embroidery machine. I never sewed on the 8500 and haven't sewed on the 4000 either. I'm too busy embroidering on it.
To be honest, if I had it to do all over again, I would look for a machine that is not a combo, one that does only embroidery. I have a Bernina Deco 600 and 2 Bernina Deco 650's, they do only embroidery, and take 2 sizes of hoops, and stitch out really nice. They're not as advanced as the Innovis but they are classroom machines and more than adequate for my students.