A solid expansion to an already excellent First Person Shooter.
Gameplay is still tried-and-true Half Life 2 and nothing much changes except for the dynamic duo system where you have a buddy with you at all times covering your back; it's a system that works and would've been nice to see in Half Life 2. Physics also play a large role in this expansion, basically in the sense that the already heavily modified Source Engine has been slightly tweaked to make up for new additions in physics seen here. Such is the case when you must stack wood to make a walkable plank, or to weigh down a broken pipe so you can cross; it's context like this that can keep a game fresh and memorable.
Sound also plays a big role in here too; you can hear Combine troops telling eachother: "Man down, man down!", "Grenade! Displace!", things like that can add a different realm into your gameplay experience that you see only in games like F.E.A.R., Call of Duty, and Rainbow Six. The musical score is also something to behold, but it also get's repetitive in its same sing-song notes and instruments.
A few other things that can seperate this game from the rest of the pack is the induction of the commentary system, with this the gamer can peer into the eyes of the developer and listen/watch on what went on to make a particular scene, model a specific object or just plain ol' ramble about what Randy drank while giving specular mesh highlights on the Combine biped. While it certainly is inventive, the specific talks can become boring and unthoughtful to the regular, un-tech-savvy user; the layman.
All in all, the game can be tedious at times with it's delectable puzzles, it can also be considered to be one of the best, short expansion packs to date on any console, PC or otherwise.
If there is one respite it's that there will be 2 more sequels added into the mix.