Has more of the unique level design of HL2, but not much.
Episode 1 gave fans of Half Life 2 and the source engine something we all wanted: more game to play through. Not a true expansion, it simply gave more interesting levels, dialogue, and even a few enemies to shoot through. The levels sport more of the quirky and unique level design found in Half Life 2, including a few neat, object based puzzles to get through, some interesting firefights, a couple of familiar, fun battles with gunships, and plenty of dark, underground places infested with zombies and Headcrabs. There's also a new twist added to this batch of levels: you fight alongside Alyx through almost the entirety of the game.
For what is essentially a 20 dollar level pack, it has about as much bang for your buck as a box of wet fireworks. Most players beat the episode in about 2 hours, at least according to Valve's recent statistics regarding play time.
And really, the game feels like a project that was procrastinated on until the last minute. Many portions of the episode are just familiar reruns from Half Life 2: for example, the first weapon you get is the super gravity gun, and you yet again use it to tear through enemy troops. One level is just a very short strip of obstacles leading into a train station, but you have to slowly lead four groups of people through it and into the train station, one at a time, before you can continue. Several portions of the game have a rather boring and annoying process where you have to 'plug up' a series of antlion burrows before you can continue, by putting cars over the holes with the gravity gun. It was interesting at first, but by the end of the episode you've done it 4 or 5 times and it's just another long, tedious process necessary to continue.
The constant presence of Alyx, while adding some interesting dialogue and a new dynamic to the game, quickly erodes your patience and goes from interesting to a hassle. Her comments are odd and out of character and really disrupt the mood of the game, especially during the tense, scary portions where you fight zombies in the dark. While an interesting addition, a bit more 'time off' from her during the game would have been fitting.
Fans awaiting answers to their questions from the last game's story, or at least an approach to some resolution for what has happened, will be sorely disappointed. There is barely any plot progression, and nothing that happens throughout the entire episode is unexpected or revealing.
The Bottom Line
Episode 1 is not an expansion. It adds no new weapons, only 1 new model, and doesn't touch on the story at all. I knew this before ever playing the game, but I was still perfectly willing to pay for some more high quality levels and play time.
But, for what is essentially a map pack with a 20 dollar price tag, there is hardly any content. 2 hours of play time on what is mostly old, re-used gameplay concepts and map design is a pretty raw deal. Fans should be perfectly willing to skip this title to save themselves a buck. The low amount of content simply cannot justify it's price.