Regardless of where he learned what he has learned, he presents his knowledge and experience in an easily accessible way. He uses simple language, diagrams and pictures to illustrates steps in his training. His books are affordable and widely available. Other trainers, perhaps including those he learned from, tend not to be so easily accessible and easily understand.
I've got to say, I don't think his franchise is as money centric as Parelli. I've read a book or so of Roberts and he doesn't encourage you to purchase anything from him, there aren't any levels, or special equipment. You don't need to attend a clinic to understand him.
I like him to an extent. I think the Join Up is pretty neat, and some of the ways he discusses training and horses gives you a new a perspective (at least it did back when I was a kid). I've met him briefly and he seems like a nice guy. I've met Parelli too but much preferred Roberts. Beyond basic work though there doesn't seem to be a lot there. Basic work is important, it helps to build the foundations for later training but I do not feel that it is a "complete" system by any means.
Go ahead and try it with your horse, its pretty simple :) Be aware though, at least in my experience, it is difficult to achieve the join up outside of a roundyard. I would even say "near impossible" for the inexperienced trainer and average horse.