A strong multiplayer experience that is fun for a little while. Also it's free.
The premise is pretty simple, you are one of 4 marines who's tasked with fighting an alien infestation. Not much of a story, but it plays out as an top-down isometric shooter so story is really the last thing the game needed anyway. As long as aliens run at you, who cares? As a free game, minor sacrifices are expected, and story is one I'm willing to ignore.
The game channels a lot of different titles with it's game design. The best way to describe it's gameplay is a top-down version of Left 4 Dead, with a spoonful of Team Fortress 2 class system and turrets, and some Natural Selection on the side. Currently there are around 7-8 levels, each managing to be relatively unique, while maintaining the wrecked space colony look. Much like left 4 dead, you get climaxes and hold outs, though the game offers some unique twists.
The class system allows for some pretty unique builds, and you can develop a very strong or very weak party based on your balance. Players can drop turrets and heal each other and resupply, and allocating who plays what role in advance can make the difference. In fact, there are certain parts you can't do without a tech class. There is also a leveling system akin to Modern Warfare, but unlike that game unlocks are considerably different than the starting weapons, which might be good or bad depending on how much value you put in balance. The game play itself feels fast paced and frenetic, which is especially great with 4 players. Many enemies specifically require 2 or more players to overcome, and there is strong satisfaction to beating levels and enemies with an intelligent team. It's not completely without issues, however.
On several occasions I noticed the melee system is a bit fickle with what actually counts as a hit. You need to be dead on the target to score a hit sometimes. Also the game plays with the 3rd dimension in a top-down shooter, and mostly it works well, but sometimes you might waste ammo on enemies you can't hit because of the angle. The floor or a beam could be in the way. Most of the time though the gun play is fast and works well. An active reload system from Gears of War is a nice addition, but doesn't really fit perfectly. In Gears the action is calculated and slow paced, while in this game it's fast and chaotic, so focusing on your reload meter while running from 3 aliens isn't exactly an option. It takes more practice than I'm probably willing to afford to this game.
Difficulty in the game is pretty reasonable. If you have a competent team, you could probably blast your way through the campaign on hard in an hour of play. Insane is where the real challenge comes in. I've only played a few levels, but without proper planning you get pulverized over and over. It borders on trial and error at that level, but with careful movement, balance, and planning, it is certainly possible. Of course like any co-op game, it can be frustrating on any difficulty if you pull a team of flunkies along, and AI friends are just plain brain-dead.
Presentation-wise, the game is reasonably proficient. Lighting is decent enough that the game adds flares and flashlights if you want to light up areas, and from the top-down angle textures and animations seem pretty solid, especially considering it's free. Sound design is solid throughout, though hearing call outs like reloads can be a pain without adjusting sound levels. Gun's have a strong weighted feel, and impacts from enemy hits shake the camera violently. Slow-mo deaths and zoom ins are pretty cool too, but it can mess you up if you're trying to avoid aliens while a friend dies.
Much like the xbox live game Aegis Wing, you can't really go wrong with free. And also like that game, it offers a pretty solid experience that holds up well as a free title. I doubt it would compete against many priced competitors, but compared to free games, it's a good game that's especially fun to play with friends, especially because you need the meat shields.