Mary has just recently gone over two of my YA books, and I will have her do my next RS as soon as the revisions are done.
What I can tell you is that she's very thorough - to the point that she read the first one over a second time as she was concerned she might have missed something. She's not terribly fast, but I'd exchange speed for quality any day. What we did was have her send back a few chapters at a time as she went over them. In this way, I could make corrections and ask her about her suggestions at the same time if I didn't understand. I personally think the commas are out there to drive me nuts, and Mary keeps them all in place.
