I have not used one, but I've done some research on them. For what they are, I don't think it's worth it unless you've got a big pile of 80% lowers sitting around and don't already own a router and a drill press.
The GG is $1000+, each lower is $50 and they ran into ITAR issues or something that prevented them setting up the forum that they were going to have.
If you get one, I'd love to hear how you like it.