Half-Life 2: Episode One is a FPS and action game created and published by Valve Corporation.
Why it Rocks
It retains the same elements as Half-Life 2.
Improved graphics, compared to Half-Life 2.
Fun combat with the Combine.
The game starts somewhere near the citadel, which is a very good thing.
The Super Gravity Gun appears again, and yes, you can still smash Combine soldiers like you did in Half-Life 2.
A new enemy, a.k.a. the Zombine, is introduced in the game. It is a Combine soldier which is controlled by a headcrab, which can hold a grenade until it blows up.
Awesome voice acting, just like its predecessor.
Fun and challenging gameplay. There is an achievement, called "The One Free Bullet". To achieve it, you have to complete the game without firing more than one bullet (crossbow bolts are counted as bullets).
Isaac Kleiner's speech is interesting. Take a listen.
The game is really immersive.
Some unused ideas from the development of Half-Life 2 have been reused seen here for the Episode's plot. Especially, the jaw bone from the Basic Zombie's model as seen from the the leak, being reused for the Zombine.
The game is very short, compared to its predecessor.