A couple questions you should ask yourself before you play this game, although you may not have the answer until after you are done.

What do you like to do most in a game?

1. Be the marine blasting your way though invaders?
2. Fly the space battle blasting your way through a space armada (perhaps an exaggeration)?

This is an interesting free mod on the source engine (requires Half-Life 2 or Counter Strike: Source to play), space battles and then foot battles, or straight foot battles depending on the mode of play.