Even better than Half-Life 2.
Gameplay:
Pros - First, you get to have your sidekick Alyx along for pretty much the whole game. The guns are reasonably balanced. First, the AI in this game is revolutionary. Enemies and friendlies will be able to take cover, blast some Combine gray matter out and help you out whenever they can.
Cons - There is occasional locking up at complex parts.
Graphics:
The graphics are absolutely cutting-edge. The shaders, written in High Level Shader Language make the scenery look beautiful. The very professionally made textures, along with high-dynamic range lighting make scenery look really good. The engine is also very scalable, which allows me to play on a really crappy graphics card.
Sound:
Pros - The sound is reasonable for an environment like this. With distance, the sound is modified so that it sounds far away. This adds to the realism of the graphics.
Cons - The sound can stutter at times.
Value:
The game is very good, but it's too short.