Half-Life 2: Episode One is the continuation of Half-Life 2's story. It picks up right where Half-Life 2 left off. It's mostly the same of what you already had in Half-Life 2; fun combat and interesting maps. I feel as if in Episode 1 the developers were brave enough to build more creative maps that demand elements which were mostly forgotten about in the original Half-Life 2. Like, for example, during the first part of this game you have no proper weapon (i will get to this again later), and you have to navigate a dark tunnel full of zombies alongside Alyx. In Half-Life 2 you never really had to use your flashlight, but now you need it to bring light into the darkness and enable Alyx to kill the zombies for you.
Besides this there a two things i'd like to point out which i disliked:
1) This game is a lot more linear, or at least it felt this way to me. In Half-Life 2 there were often split paths you could go down and explore, while this game is basically one straight line. This is further solidified by the fact that this game has wayyy more "wait whilst the story is being explained" type moments. More often than not you have to listen to Alyx talk while she does the important things, like fiddling with the computers to get you going. I feel like the orignal Half-Life 2 better integrated these moments into the gameplay loop.
2) Is the aforementioned weaponry in this game. There is one achievement in this game called "The One Free Bullet". To gain this achievement, you have to play through the entirety of Episode 1 whilst only firing one singular bullet (you have to shoot a lock off at one point). And like mentioned, the only weapon you have for the first few chapters is the gravity gun. I suppose this game was heavily built around this achievement, which, on one hand is pretty cool because it forces you to think of creative ways and use the physics to your advantage, but on the other hand is rather not so cool because Half-Life has a great gun feel and choosing to use the gravity gun should be a decision the player makes. More often than not i found myself cornered and beaten by zombies, because i couldn't properly defend myself with the explosive barrels without killing myself, too.
In the end, Half-Life 2's Episode One is not a must. If you enjoyed Half-Life 2, and particularly want to know how the story goes on after the cliffhanger, then you should get this game. If you don't care about the story, then don't.
Final Score: 8/10