Just to comment, I was in the same boat as you guys with the upgrade to 10, but I tell ya, it was the best $10 I've ever spent, gmod 10 is worth more than the entire Half Life game series combined in my opinion.
Generally, maps will work with older versions of gmod depending on what is in them, if they contain Half Life EP2 content in them, then they won't. New LUA code will not work at all in any older versions of gmod.
