A beautiful mix of Real Time Strategy and First Person Shooter/Combat.
The two sides are completely asymmetric, marines are mostly focused on ranged weapons. Where aliens are mainly melee combat. I always liked both; ranged vs melee combat, and the two genres; RTS and FPS. This game is mixing all this together, and making it into a very team based game, where technology and resources play a large role.
And you can definitely feel that when playing it. If you're playing publicly, you will often hear players talking to their teammates, planning what to do in what specific situation.
Also, the game have good odds of becoming a competetive game. There are already teams playing now, in the beta. And I expect number of teams to explode when release happens.
The game is played 6v6 in a competetive setting, similar to CS 5v5, but with one extra player on each team, since there are commanders on each team in this game.
My suggestion to you: Buy this game if you haven't. Great modability too.