I made the naive mistake of not buying this, thinking "Half-Life 2 might not be so great and I may not want the episode packs; Team Fortress 2 looks silly and Portal looks weird. I'll just buy HL2 and save money."
Well, two months on and I'm sorely regretting not buying this because I have since bought EVERY game individually, and regard each one as one of my all-time favourites.
Half-Life 2 (and particularly the episodes) are beyond explosive and it's very playable despite the age. I never once felt the game was 'dated', due in part to Steam still updating their older games - I was impressed to see HL2 making use of my 64-bit processor, which is more than many brand new games do. Oh, and Team Fortress 2 is the *only* multiplayer game most of you will ever need. It's as fun as it is addictive.
Do not knock Steam: as well as allowing you to play your games without inserting the DVD, Steam automatically patches and updates your games. It is an absolute lifesaver and I now buy all my games over it.
If you're a big online gamer and don't already have Counterstrike, and feel that TF2 may not be enough to satisfy you, consider buying the Source Premier Pack over Steam instead. It includes the Orange Box in the price, but throws in a bunch of Source games too.