I played the modification of Half Life, Counterstrike, and was amazed at the realism.  I played it at my friends house and only thought of Half Life as a barrier between me and counter-strike, I played counterstrike for a couple nights, and sucked it up because I hadnt even tried Half-Life, so I thought Id just use Half Life as a trainer for Counterstrike, again not expecting much. I was sucked into the story line by the second chapter.  My only complaint with the game is that it didn't rely much on me helping out the other scientists in sticky situations, but Half Life 2 is coming out and Im sure that that will make up for the badstuff.  The game took me almost 30 hours of game play to beat on normal settings, and except for like 2 parts in it that i got frustrated, they were the most intense 29 hours of my life. (The two parts that weren't fun were in the trams, and killing the nihilith or whatever its called.  Take my advice and buy this game right now, its only 17 dollars, id gladly pay twice that for just the single player, buy it for both counterstrike and the singleplayer though.