Is it worth the $20? Heck yeah it is!
Graphics - 10
Valve still takes full advantage of the Source Engine. The environments look great, and still feels natural like in the last game. The textures still look great, and are put together well. And the models look great as well.
Sound - 10
The sound is amazing. The ambient sounds are still as awesome as the last game. I mainly love the Sound FX of the Zombine. But let's talk about the music. THE MUSIC IS JUST SO BADA**. I mean, wow. The music has some great rock elements witch really pump up your adrenaline, and makes you want to kick some butt. And the voice acting is still superb, Especially Alyx.
Value - 9
Now, I had to give value a nine, mosty because I had most of the muti-player games on my Steam account. But really, who would wanna buy it just for the multi-player?
Reviewer's Tilt - 10
Overall, if you want to know what happened from the last game, and are looking for a great game for 20 bucks, look no further. I highly recommend this game.