This is Valve's first try at episodic gaming and it takes place right after the end of HL 2. Obviously Gordan Freeman have survived the game and are now in the Citadel, which has become unstable, and they must flee before it blows up!
The game also included and audio commentary, which adds talk balloons all around the game that when used, comment scenes and many things about the game, they also make you invulnerable during the commentary, but it never really cheapens the game and you can always switch it off when an enemy is near...
You see the graphic improvements from the previous game, with HDR lighting effects and getter facial expressions and since Alyx will be with you during nearly all the game, her AI and expressions have been improved and she doesn't seem robotic anymore....
As you try to escape from City 17, you'll visit new areas and meet new enemies, the Zombine, zobified Combine soldier which are quite dangerous since they carry grenades, they don't throw them, they just activate them and blow themselves up while chasing you!
As with all episodic games, the length is far shorter than the normal game, but they also cost much less that a full game.
But it wasn't that short, I listened to all the comments and didn't get stuck and I beat it in about 5 hours? The story was pretty good as always so I enjoyed it a lot! The action was quite good too! I fought about 3 bosses in the game, and although the bosses weren't new, they did provided you with a new way of fighting them! Oh you finally get to meet face to face with Stalkers too!
So if you liked Half-Life 2, try to get your hands on this, although who knows? Maybe they collect all episodes in one DVD in the future?