It is insanely great. My favorite mod. It also has decent mapping tools and capability and features a particle system (Oooh, aaah!) which looks great for the HL engine.
You need to have Steam in order to play NS in its newer version (more levels and such and more servers). So, version 1.1.2.0 or whatever the most recent Steam build is. Autoupdates via Steam will take care of that.
They have a new version, beta 3 build 4a, so make sure you have the most recent installers.