Over before you know it, but still very satisfrying.
First off, as I always like to do, get the aesthetics out of the way. The game still looks great even a couple years after the original HL2, and the addition of HDR lighting into the mix definately helps upgrade the engine a little. The main problem here is that though you will go through 'new' areas, a lot of the environments feel the same as the original. That's not to say they look bad, because they still contain all the love and detail that HL2 contained, but it just gets a little repetitive. In fact, you spend the whole first part of the game going through the same exact place you went through in HL2's climax, except it is a little more destroyed this time around. Still, this game does look good, it just could have used a little more variety.
The game sounds just as magnificently as HL2, with [possibly] better voice acting then before and sound effects to match. Just like HL2, this will be a game you will want to turn up.
Considering how much Opposing Force had added to the original HL, I personally expected quite a bit out of this expansion for HL2, I expected new weapons, new enemies, an interesting story and some nice additions to the multiplayer. The only part of that that is truly fulfilled is the story.
There are NO new weapons, and in fact one of the weapons from the original is not even available in this expansion (the pherapod). There is only ONE new enemy I can remember from the top of my head. Granted, this new enemy does make certain encounters interesting, and it is a nice new enemy, but that is the only one there. There is nothing new in the multiplayer, in fact it includes HL2DM and Half-Life Deathmatch Source, both if which you already probably own if you have bought HL2 outside of Steam.
So with all these complaints, what is it that makes this game fun? The action. The game basically takes the fantastic action from Half-Life 2, makes it a bit more intense, and throws you back in the mix. The action in this game is fantastic and well done, and though there is no real last boss, the climax of this game will definately have your heart pumping. There is not as much of a 50/50 split in puzzles/action like HL2, but rather more of a focus on the action and less on puzzle (I'd say something like 70/30). This isn't a bad thing, but it does make the game end a lot more quickly than it may have were there a few more puzzles.
To add to that, the character development (specifically of Alyx) is absolutely fantastic in this game. On top of that, the continuation of the story is taking a very interesting direction.
Because the game is short, you will likely take advantage of the included Developer's Commentary, which has all sorts of interesting tidbits about the game, but other than that, the only reason you might come back to this game is to replay it before playing Episode Two (and Three whenever that comes out). The multiplayer is great, and the new Half-Life DM Source is nice since you finally get to play the original multiplayer in the source engine, but that also means all those mods you used to play need to be remade for HLDMS. To me, the only new thing is Episode One and the replay value is really based on that.
Don't take it the wrong way though, this is an immensely (though short) satisfying expansion to one of the best PC games ever made, and that is no small feat. If you have played HL2, this is a must, just don't be surprised when you end up beating the game in 1-2 sittings. Enjoy.
Graphics: 9/10
Sound: 10/10
Gameplay: 8/10
Value: 7/10
OVERALL: 8.5/10