The game is fun. It is like Left 4 Dead meets top down Contra. You have 4 classes, soldier, special weapons, medic and tech. Then each class has 2 different people with slightly different "stats". You go through levels fighting off aliens and completing objectives to move on to the next level. You get level ups and access to different equipment. Between each level you can change characters/equipment load out.
You need teamwork to win. Almost every level needs one tech character as the tech is the only one who can open certain doors. If the tech dies, it is usually automatic game over. Special weapons are also sometimes needed for the flame thrower to complete certain objectives. If you need one of these classes the game will not let you enter the level until the right class/equipment is obtained. The soldier is basically just good with weapons, placeable turrets, grenade launcher etc. The special weapons is the only one who can use the flamethrower which is needed and also gets miniguns. The tech is needed to open certain doors and panels. People can also equip welders so you can weld shut doors after you go through them, to stop the aliens from coming in from behind. The medic has group healing and a healing gun and is needed to cure the crazy infestors, which can 1 shot anyone in the game. Generally speaking you will have no problems with a diverse team of 1 of each class, but you don't need certain classes for each mission.
I recommend a medic on each time, and I pick the chick for her better infestation healing. The infestation basically puts a damage over time on the character and it must be healed off. I successfully healed off an infestation solo by using 3 personal med kits that I found in the same area. You can also throw down an AOE heal kit or use the heal gun medics can get later.
The game has lots of fun action, cooperative gameplay ( up to 4 at once, recommended to have 4 above normal ), some decent achievements and can have some pretty crazy difficulty where you can die easily in 1-2 hits. There are some unique aliens too, such as an alien with a shield in front. He will basically "aggro" one person and charge and everyone else has to get behind the alien and shoot away.
The game is pretty short and you can probably do the whole campaign in about one hour, maybe two. The game is fun enough where you will probably want to play at least on a few difficulties, even just to try other characters out. The game is very Left 4 Dead feeling, but it isn't as bad in the multiplayer. It is harder to grief people. Sure you can shoot them, but you can't do much damage at all honestly.
I would strongly recommend for a free download. I would even pay a good $10-$15 for the game, but not much more for such little content. For a free game though, very impressed. If more campaigns were available as DLC I would buy.