It's really three short games in one nicely wrapped package...
The Marine campaign is a frantic, high-octane rollercoaster that offers paranoia and jump scares as Xenos hunt you through an abandoned colony and spaceship.
The Alien campaign is the exact opposite: a methodical stealth game in which you are better aided by your ability to learn the labyrinthian level layouts and strike quickly and unseen than charging headlong into the fray like the Marine.
The Predator campaign is a fun, even relaxing romp through a jungle and military base in which you enjoy all of the perks of being the Ultimate Hunter: a variety of unique weapons and gadgets, free healing every 30 seconds, different vision modes for tracking all types of prey, and so much health that you can melee a small army of Xenos dead before dropping yourself.
In addition to the levels offered by the vanilla campaigns you can also unlock a second campaign for each of the three main characters by completing their respective campaigns on the hardest difficulty (Director's Cut). These new campaigns allow you to play through certain Alien levels as the Marine or Predator, certain Predator levels as the Marine or Alien, etc. New items also appear such as the jetpack for the Marine, which might leave you briefly wondering if you aren't playing Tribes.
There is a multiplayer mode, although these days there doesn't seem to be much activity online. If you find that you're convinced to buy this game and enjoy the gameplay, you might consider seeking out some of the AvP fans on Steam or encourage your friends to give the online a whirl with you.
Pros:
+ Good amount of content for all characters
+ Soundtrack and visuals create three unique atmospheres for each of the campaigns, ranging from frightening to empowering
+ Replay value in the form of secret levels and cheat codes that can be unlocked
Cons:
- The Alien's controls are challenging to learn
- The high difficulty will put off some players
- Many of the levels are maze-like and offer little hint of your next objective