First of all, this game is great and free!
So at least give it a try yourself.
It's a great coop based game. To take the best out of the game you have to work together and play your part in the mission.
The official campain is great fun and you can play it over and over again, trying different ways to beat it.
Join ASB, Riflemod or chickenscoop servers to have even more variation in Alienswarm.
Sadly, this game does not have a workshop. That means you have to manually install map packs when you want to play other maps than the default 7.
To get most of the game with additional settings, pvp modes, 8 player coop, new weapons, challanges and pre installed maps get Reactive Drop. Though make sure to join the official RD-group for scheduled events.