If you are a skeptic of episodic content, then Half Life 2: Episode 1 will turn you into a believer.
The graphics in this game are outstanding. While you need a high end computer to turn on all the eye candy, even a lower-end machine will churn out some attractive visuals. Though the visuals are beautiful, the real thing that makes the source engine stand out is the fact that it is so scalable. I can run pretty much everything on high with an ATI Radeon 9800 Pro.
Gameplay is pretty much identical to Half Life 2, except the enemies are a little smarter. That said, they still seem like they would not score so well on intelligence tests, as they will often stand in exposed positions or charge directly at you. Additionally, having Alyx by your side offers a great sense of camraderie that no other Half Life game has ever delivered. Additionally, some of the weapons are rather annoying to fire, as they don't feel very realistic; accuracy is seemingly not affected in any way, even when sprinting. Despite these flaws, there are some really great moments of gameplay. Once or twice during the game I felt almost like I was playing Brothers in Arms because of the way I had to utilize cover.
Sound in the game is very good. The different weapons all make satisfying noises depending on their properties. Personally, I would like to see a little more in the way of a soundtrack; the few times that the Half Life techno style songs play in the background, the game is given a more cinematic feel. The voice acting is excellent, though very rarely there is a questionable bit of dialogue.
As you have probably heard, it will not take you very long to play through this game. To make matters worse, the beginning and end feel very rushed, as if the developers could not think up of a good idea. To beef up the value, there is a very well implemented commentary track, which, if you are very interested in that sort of thing, could conceivably double the length of the game.
It is very easy to reccomend this game. The companionship brings a refreshing touch to the half life series while the source engine makes the game extrememly immersive.