You are right. From my POV their problem is that you cannot use them for international projects (non US customers cannot use ACH; non us businesses cannot get their money out of Dwolla)
In so far as large use cases, they have some really good traction in the bitcoin market. I believe that they are the go to for transferring USD in and out of MT. Gox.
I've heard both good and bad about them, but they don't have large use yet.