I didn't do Of Mice and Men, but I had CGP for The Crucible an York Notes for To Kill A Mockingbird. I found that the CGP was far far far more useful. York Notes is very basic. I'd suggest reading the reviews on Amazon.
I would also recommend MrBuff's videos on youtube if you haven't checked the out yet. He has a playlist dedicated to OMAN and A* analysis for it Personally, I have the York Notes guide, which is decent, but there's only basic analysis. There are some useful exam techniques in there though...like using embedded quotations and stuff.