AvP2 is a game which brings justice to both Alien and Predator fantasies.
Let's start with its graphics. I'm really impressive by the graphics for a 2001 game using the LithTech engine. It's even better than the first Halo IMO. Take a look at the screenshots on this site.
Now, sound. The sound in the game is from both Alien and Predator movies not including AvP movies. They did a great job putting the sounds into this game. I really like the sound of Predator roaring when you collect skulls and Alien screech when its shot.
The gameplay is really good too. Being able to play as all three species, Marine, Predator, and the Alien. All three have their special abilities. For example Marines have the hacking device which is used to get through lock doors, the Predator can cloak itself so that prey can't see them from far, Aliens can wall-walk and have very fast speed. I personally think that playing as the Marines for the first time really can make you wet your underwear. The others aren't really that scary, they just startle you. The story of the game is great, cut-scenes are done nicely.
Single-player is very addicting... and so is Multi-player! There are still many people who are playing Multi-player so you won't be alone!
Overall, I say that Monolith has done a great job with Aliens versus Predator bringing justice to both Alien and Predator fantasies. Hope that there will be a AvP3 with the same stuff as AvP2. This game is very rare and if you find it buy it right away!