Alien Swarm is a game forgotten by both Valve as it's developer and the community. A game that had potential, but comes out just feeling unfinished. The concept is good enough, but it probably got canceled and someone just decided that it would be shame to leave it, and put up what they had at that time. And well, if it wasn't good enough, people didn't like it enough for it to become good thanks to the community.
The only thing that truly keeps this game alive, if you can even call it alive, is the fact it's fully Free to Play. No payment on Steam, no microtransactions, nothing. You can join the fray at any time and see what they got in store for you. However, it still features a level system - the more and the better you play, the bigger level you are and the better equipment you get.
This unlocks one of the games biggest advantages - a very smart equipment system. You can choose between four classes, with two characters for each, both with completely different playstyles. Then you get to choose your equipment, two weapons and a utility. Of course, all four of the classes have access to equipment available only to them, as well as access to certain shared equipment. For example, the Medic is the only one capable of carrying the Medikit or the Medigun, the Commander is the only one capable of carrying shotgun, the Engineer is the only one capable of carrying a rifle with autoaim etc. The game then features more than 40 weapons and about 15 utilities (if i recall correctly). This allows you to play the game differently every time, but thanks to the level system, certain weapons will always be better than others, even though the game does try to balance them out.
The gameplay is then based around a co-op mode for four players. You (and hopefully a team you can trust) are sent to a colony gone dark to investigate, and *drum roll* fight off aliens there *badumm tshhhhh*. However, you are alone. So you need to conserve ammunition, use medkits and cooperate while holding off the bugs in order to survive, and fulfil objectives along the way. All in a top-down Source game. That means the SDK makes the maps quite interesting, counting in things like fire, liquids or destructible enviroment, even some in-game animation parts along the way. This all creates a feeling of being swarmed and overrun, yet deeply dedicated to ending the mission succesfully and without casualities. Talking about casualities, it's worth noting that there's friendly fire ON by default. Watch it.
Probably the most notable change is the Source SDK here. If a big community would be on... millions of maps. IT's just that easy to make them. Tile based creation and Source modelling = Map. I tried it out myself, it took me exactly 76 minutes to recreate the first level. It's just sad it is not used.
Also please note that Win 8 is not supported by this game. It will run after you install it, but Win 8 will screw up it's files and you'll have to reinstall sooner or later.
As to my recommendation, I can say that this is a game you will enjoy at first, and then become disappointed by later. Play at your own risk.