Had a mapping jam with a friend today and decided to use the opportunity to try out Hammer for CS2 - I set out to make a single-bombsite Defusal map, but the end result became more of an aim duel map. We prepared by watching through Eagle One's introduction video, which was very quick - I would recommend pausing and rewatching sections liberally - but it was a good intro and covered the basics well.
Once you get past the smorgasbord of tools, blocking out levels in Hammer 2 was really nice: you can extrude things in SketchUp style very quickly, but you can also block things out in Trenchbroom style if need be.
The major speedbump we ran into is that Workshop maps still need mod approval even if you're just testing them out with friends, so setting up a dedicated server is a good idea if you're going to playtest.