Well...Where to begin.  This game is excellent.  That's basically all I need to say.  It's main game has a nice plot, your a scientist whose expieriment went wrong and you must do some stuff...AND SHOOT STUFF ALONG THE WAY!!!  Most enemies are Smart, and tactically try to kill you, and the "Puzzles" are fun too.  Plus, Deathmatch is fun, and all the "mods" by other people make the game infinately replayable.