Wow, 149 comments and not a single one modded up about actual gameplay?
I'll go first - it's a pretty fun game. It's a squad based top-down shooter based on the Source engine. Multiplayer using either shared or dedicated servers. You choose one of 4 roles - Commander, Special Weapons, Tech, or Medic. There can be more than one Medic for example, but most missions require at least one Tech to hack open doors.
You choose your weapon load-outs and characters/roles and enter the mission. You can carry two main weapons and one off-hand weapon. Some of the main weapons are unique to the character type - for example only Special Weapons can carry an auto-gun, and only a Medic can carry a medic gun, and some are available to all players. As you complete missions you gain XP and levels, which unlocks additional weapons. The levels are account specific, not character specific - for example I can get to level 9 playing as Special Weapons, which unlocks the Medic gun, and then I can switch to Medic so I can actually use it.
The controls are fairly simple - W,A,S,D for up/down/left/right, and the mouse controls aim/firing, but since it's top-down you do have to aim in 3d space - you can aim close or far away. Friendly fire does happen with all but a couple weapons so you need to be careful not to hurt your teammates.
Aliens swarm from almost everywhere - I guess that's how it got the name. They do come from creative places - climbing up from hatches in the floor, jumping down from the ceiling up above, climbing walls and tunnels. You can never be sure where they will come from. The missions seem to be mostly linear objectives, like gain access to this area, go here, destroy the alien biomass, escape, etc. and are filled with choreographed sequences of alien battles to keep you on your toes. If you stay too long in one area you will start to get random alien attacks as well, so it's not 100% choreographed/scripted.
There are only about 7 or 8 missions in the game, and a group of friends and I were able to complete all of them on Normal mode in a few hours last night. We got to level 10 or so and unlocked a lot of weapons in doing so. I guess now the only thing to do is play it on Hard modes or wait until they release more maps/content.
Some standouts: Your off-hand weapons are very unique and can be used for tactical advantage. For example, you can get combat flares that increase auto-aim effectiveness when dropped, hornet swarm missiles that are heat seeking, even adrenaline that makes the game go into slow motion - which has to be the coolest effect. In slow motion everything goes into The Matrix-like mode where you have tons of time to aim and line up your shots. The game also does it at certain times on it's own just for dramatic effect.
Conclusion: A fun but short game - it will be interesting to see what type of content the mod-community creates for it. The squad-based tactics can be surprisingly deep if you have a good group of players on voice communication. Definitely worth a download for at least an afternoon or evening of fun with 3 friends.