Tremulous

The Good
The setting and atmosphere were nicely designed. I can't help but to think of the "Alien" movies with Sigourney Weaver. There is a good difference in playing style between humans and aliens. Also, both humans and aliens have different player classes to choose from. Humans are able to build different structures, and aliens could evolve into more powerful creatures. The most fun you could have is by becoming a more advanced alien and executing the attacks.

The Bad
It gets boring quickly. If you play as a human, most of your time will be spent defending your home base against the rushing alien attacks, and if you play as an alien, most of your time will be spent rushing the human base. That's it. There is really not much else to the game play. It also doesn't help much that the weapons the humans have access to aren't very interesting. Some of the structures you are able to build as a human don't serve a real purpose, and are mostly ineffective against the alien rushes.

The Bottom Line
This is an average FPS with some strategy elements which will keep you interested if you enjoy the presentation and atmosphere. The nice concepts are there, but their execution is just OK.