I found Greg to be really insightful about writing, especially about a writer's unconscious motivations, particularly when things in a piece aren't quite working. I think his critiques were thoughtful and engaged, although I wish that he would have given more concrete, specific advice in one-on-one meetings. In these conferences, it really felt like the onus was on us to discuss our own work, and while that can be useful, I thought he was a little too passive in the whole thing.
Workshops were usually useful, although (and this could probably be said for many writing profs), I wish he had been a little more active in guiding them. Sometimes, it wasn't quite clear whether we were calling on each other, or he was choosing who was to speak. Overall, though, I thought his central teacherly conceit (being aware of "experience" vs. "thought" in a piece, and how they're connected) made sense. I'd recommend him.