Is it any good?
For someone who has never played Alien Swarm, give me a summary of what its all about?
Alien Swarm is a 4 player cooperative game. Less than 4 players can play but I don't much enjoy playing with the bots (though you can switch your control to take control of a specific bot, if you wish).
It is at its best online with voice comms on a higher difficulty (Normal difficulty is fine for an introduction if you know vaguely how to play).
It is a mostly top-down game, though this release has added in just a touch of isometric perspective. Personally, I like this view more though it does slightly hamper the view of aliens coming from the bottom of the screen.
The players pick characters that conform to one of four classes, and there are two characters available in each class. The classes are (loosely) leader, auto-gunner, medic and hacker. If they were in the film Aliens they'd be Apone (Sarge), Vasquez (Wildcat), Dietrich (Faith) and Hudson (Crash).
The missions tend to be fairly simple: go here, turn this on, go there, burn that, escape. They're complicated by aliens, of course. Aliens tend to arrive in batches, depending on the map, but Alien Swarm is notable for its set pieces, usually based around hacking. One marine hacks some console trying to open a door or something while the others try to keep the aliens off his back. Or the marines are all on a lift that is descending while aliens jump down from above. For me, this is where Alien Swarm is at its best and works brilliantly coop.
There's different equipment that is chosen before a mission, some of which can only be equipped by certain classes. In contrast the original release on the UT2004 platform, this version also includes a simple levelling system that allows access to greater quantities of equipment; the basic equipment is actually pretty good and the more advanced stuff tends to present more ways to fuck up and kill your team mates.
Personally, I bought UT2004
solely to play Alien Swarm. That this release is free is totally awesome.
If I had one criticsm, it's that the campaign is a bit too short. This will doubtless be remedied by the extensive availability to mod and map for Alien Swarm.
Is it good? Absolutely yes. Still not sure? It's free, so what are you worrying about!