First and foremost this is quite possibly the best bargain in gaming in the last 10 years. Not only does it give you a full first person shoot game but it also gives you two expansions to that game. Along with that it also gives you the game Team Fortress 2 and Portal.
I enjoyed playing through every game in this package. Half Life 2 took approx 10 or 12 hours or so to beat and then 8 total for both the expansions combined. Portal on the other hand only took around 3 hours to beat, and Team Fortress being online only, can not be beat.
In Half Life 2 you take on the role of Gordon Freeman fighting in City 17 where an alien force (the combine) has come to destroy the human race. In Portal you are a test subject for a corporation where you make use of a special gun that allows you to create "portals" that allow you to move long distances quickly. Lastly, in Team Fortress you are in a multiplayer team based shooter where you take on the role of one of many classes including Pyro, Sniper, Heavy, Scout, and Medic. You along with your teammates work together to do many goal oriented missions which include capture the intel (capture the flag) and capture the point. The new map being released in April 08 (Goldrush) adds a new mode where your team works together to push a cart along a rail to try to get it from point A to point B.
I personally had a blast playing through all of the games and I hope somewhere along the road Valve does this again, which would be completely awesome. I play Team Fortress the most out of all of the games and I think I may have created an addiction to the game. Its very fun to play.
On a side note this game is not for young children. The games inside of the collection have heavy gore and violence and it deserves the rating that it was given by the ESRB