This weekend I got to play a little bit of Valve software's new freebee, Alien Swarm. Think the feel of Aliens meets the top-down look of Diablo, and you're close. Add up to 3 more players cooperating to wade through hordes of hostile alien bugs, and you're right on the money.
The game offers 4 different classes of character to play - officer, medic, tech, special weapons - and the brief session I had showed some real promise. There were only two of us playing, so getting the right combination to mitigate the missing classes took a couple of tries, but we managed to slog through, 3 or 4 tense moments aside, to the end of the first area. The bugs seemed to spawn the same way on every playthrough, so the challenge (at least in the level we played) came more from tactical placement and weapons loadout than from managing chaos.
Graphically and design-wise, the game feels pretty modern, and its designers have curated a nice collection of SF tropes both new and old. My favorite so far is the inclusion of an active reload system similar to the one in Gears of War. Hit reload once, and then a second time at the sweet spot in the cycle to reload significantly faster - screw up, and you're stuck waiting a bit longer than a normal reload.
While I did enjoy my time with the game, I've run into an issue where it periodically hangs for a few seconds (and sound like a skipping CD). This also holds up the other players, and is disruptive to play, so it has really let the air out of my balloon. Trimming back settings has only brought moderate improvement, and the game is barely playable for me right now. Fingers crossed that an update will save me. From what I'm reading, mine is not the average experience, so you should definitely give it a go, just bear in mind your mileage may vary.