A mid arm has no feed dogs, so you have to push the fabric through. They are basically a free motion quilting machine, although you can attach binding with them. You use them for free motion and rulers and the like, not for regular sewing.
As Tothill mentioned, some people buy one then realize they would rather have a long arm. You have to decide whether you are a "push the fabric" or "push the machine" kind of quilter. Myself, I will always be happy with my sit down because I have tried both and I know that I am a "push the fabric" type of quilter.
Watson