HALF-LIFE 2: EPISODE 1 / Valve / PC
Episode 1 sees Gordon and Alyx returning into the collapsing Citadel, attempting to shut down a reactor core that's about to explode so the denizens of City 17 have time to escape. The first two levels of this add-on pack thus feel almost exactly like playing the last two levels of the main game - using the Super Gravity Gun to chuck about hordes of guards, taking tons of chip damage from said hordes of guards constantly flooding in from all directions, and trying to aim those stupid power balls.
This sets the tone for the rest of the game, which is basically "Moar Of The Same." After getting the reactor under control, Gordon and Alyx spend the rest of the game picking through the rubble of City 17 trying to find survivors and get everyone to a working train. After the samey Citadel levels, there's a samey stretch of "gravity gun survival horror" in a darkened underground parking garage, and then the final two chapters are much like Follow Freeman from the previous game, with civilian squad escorts in the rubble and launching rockets at Striders and Gunships and etc.
"Not a lot of really new content" is the main theme here. The game only lasts 5 or 6 hours at best, and much of that feels like going through the motions you just went through in the original game. The problem is magnified if you move straight from the original game to this Episode, as many Orange Box customers will do. The only major new element introduced here is the Zombine, basically an Overwatch who has been Headcrabbed, like a regular zombie but able to break into a jog and also pull out grenades and Terrorist you as well. Honestly, they add nothing to the game but nuisance.
I'd say it's worth playing overall just to continue the story and be prepped for the superior Episode 2 ... but really, so little develops here that you could skip this one entirely and hardly miss anything. Alyx and Gordon decide they have to shut down a reactor core to save all the evacuees. Alyx and Gordon shut down the core but then have some rough sledding in getting out of the city themselves. Alyx and Gordon eventually get out of the city before the core blows up. That's literally all that happens over the entirety of the game.
Ep. 1 falls into the same quandary that the official expansions to the first Half-Life did; while the levels are well-made, they re-use too much of what the original game used in terms of challenges, ideas and settings. There's a neat and exciting moment here and there, but it's counterbalanced by the derivative and annoying bits. And it all just leaves you with yet another abstract, dicktease cliffhanger in the end, intended to get you lined up to purchase the next installment.
Quuaaaid ... start the reactoor
* Visual Walkthrough
* Gameplay Video