I have a Janome 4100QDC and am pretty happy with it, although the throat space is too small for a king sized quilt. I use this machine only for quilting and embroidering labels. I have two older Singer machines that I use for piecing (a 301A and a Featherweight). The Janome pieces just fine, but I just prefer my vintage machines for straight stitches becasue their bobbins hold more thead and well, I just like them.
Drawbacks of the 4100QDC: it only has one alphabet and the font size is fixed and not adjustible. It hates the invisible thread (the synthetic kind) and refuses to sew with it, but I understand this is the case with many machines, not just Janome. The throat/harp size as I mentioned before. It also does not like to sew into larger "bumps" where a series of seams intersect, like the centre of a pinwheel, for example. But, I am not always the best at nesting the seams, either. Sometimes I have to leave the needle down in the fabric, raise the foot and move the fabric up a little bit as the machine won't sew over the bump. Mine has an automatic locking stitch that is tempermental - sometimes it works and sometimes not. It also does not have a stitch regulator, but then again this isn't a high end machine. I think I paid about $800 CDN for it, including tax. So if I'm sewing really fast, the stitches get longer. I mitigate that by keeping the speed in the medium range. I wish the display of the alphbet characters and the corresponding numbers was on the screen and not on a plastic card that you can attatch to the top of the machine in one of the thread holders. And finally, the optional quilting table (that came with my machine as some kind of quilting promotion) just rests level with the free arm and moves around a lot as there is nothing to clamp it to the machine. It has rubber feet but that doesn't stop it from sliding around when I've got a large, heavy quilt dragging on top of it. I've given up on using it as it is more of a nusance than anything.
It's a perfectly good machine, and I think represents value for the money.