Hmmm, a difficult one to discuss here. I'd like to see his books to see how he makes the stuff accessible. I almost ended up working on a variant of this, as I was invited to take my PhD under Chris Isham at Imperial studying the "problem of time in decoherent histories".
I keep trying to describe what this is about, but it requires describing so many precursors that it will take longer than I have at the moment. But it isn't as way out as it sounds. Well, compared to string theory, it's weird, but then all the great ideas in quantum gravity sound weird. And it actually addresses the problem of time, which is not touched upon in string/M-theory.
I think "generally accepted" is a bit strong. He's ceretianly not a mainstream quantum gravity guy, but he is respected and his ideas have merit... I have heard far more outlandish ideas from, for example, the world authority on black-holes. I was quite shocked :)
I'll come back to this and try to talk about it later...