Episode 1 is an interesting game that in some ways betters Halflife 2, while on others falling flat, prominent of which being the story telling.
Halflife 2's ending is completely nullified with no explanation other than "it's a mystery". Some vague hints at what's going on, but nothing beyond that. This wouldn't be so problematic, if it wasn't already prevalent in Halflife 2, instead of delivering incite, it asks qeastions upon qeastions aiming for intrigue, instead, the only thing that resonates is an increasing sense of apathy.
In the context of the last game Alyx Vance's character is nonsensical. By the end of Halflife 2, a rebellion of the local populous has left the city in ruins, theoretically thousands of people dead, starving and homeless with a looming threat of the Combine striking back in full force to end humanity entirely.
To add to this, Episode 1 immediately establishes the Citadel's core will explode, acting as a nuclear bomb removing the city and whoever is still there out of existence.
The natural response would be anxiety, fear and an overriding sense of urgency. Yet, as evident from the developer commentary Valve focused on trying to make Alyx Vance more likeable, resulting in an obnoxious upbeat optimism that is totally at odds with reality around the character. In short - the characterization is terrible.
Contrary to that though, as a game-play mechanic, Alyx Vance works incredibly well in tandem with the player. There is an interdependent, with the player deprived of a firearm for much of the games opening act in order to emphasize this. One of the best sections of Episode 1, and indeed one of the best sections of Halflife 2 itself involves the player directing Alyx Vance with a torch in an almost tower defense section against the pitch black darkness as hordes of enemies attack on mass. It's exciting, tense and generally superior to other games that mimic the buddy AI, that, rather than the terrible character depiction connects the player. Mechanically, rather than stand around occasionally popping off shots as was the case in in the main game, there is more creative thought going into how to utilize the character.
The art direction and overall detail seems upped as well, making good age of the even then aging engine. Many locations are revisited, all familiar, but there is a certain polish to them that makes them feel more defined than they where in the original game. The color pallet inside the citadel for example uses far more variation while retaining it's sterile authoritative aesthetics.
Not new much content is actually added to the game besides a "Zombine", which acts as a kamikaze enemy similar to Serous Sams headless screamers. It's a nice addition that spices things up, but the Combine themselves still remain rather lackluster in contrast to Halflife's Marines, set pieces and a tight chain link progression alleviate the issue, but it's still a problem and wouldn't really be addressed until the introduction of the Hunter in Episode 2.
Episode 1 is about 3-4 hours long, but it's quality over iniquity. It change in direction to include Alyx Vance as more of an interactive entity is it's most noticeable difference rather than the typical new multiplayer maps, weapons and enemies expected of an expansion. At the time, £19.99 may have seemed somewhat of a big ask, but now it's on sale constantly as separate or as part of the Orange box.
As a single expansion, the only major complaint is writing and a lack of content. Beyond that, it's one of the best expansions around for the genre, at least if all you care about is single player.