I have a 15", which I generally love. I'm still pretty new to using it, and there have been some growing pains, but when it's working correctly, then it's awesome. ;) The thing to recognize about the Bailey is that it's very no-frills compared to a true longarm, and is somewhat cobbled together (it's a stretched machine, technically). It has two major selling points, IMHO -- the price, and the throat space. If you're willing to put up with some initial frustration and a bit of a learning curve, you'll probably be very happy with it. And whenever I hit a snag with my quilting, I just tell myself, "This is a heck of a lot faster than hand quilting!"
One thing to keep in mind as far as cost is whether or not you need a stitch regulator -- I personally wouldn't try to quilt without one. Some people manage to work just fine without one, but the learning curve is substantially longer without one, from what I've heard.
I'm running my Bailey on a Little Gracie II. If I were to do it over, I'd probably get a bigger frame -- I'd like to be able to do king sized quilts; also, I'm probably losing about an inch of quilting space (in the throat) because the frame isn't quite big enough. It would probably be perfect for a 13", though.