It's terribly simple, with turns taking 5 seconds of "real time" each - but you get to take as long as you like to actually make your turn - the other person is notified when you've made your moves.
There's a variety of weapons, a set of commands (move, duck, face in direction X, ignore/concentrate on a particular person), and that's about it. Oh, and a few different game types - simple "extermination", dark mode (where you can't see the other team except when they're in line of sight), hostage rescuing, etc.
And I've spent large chunks of this evening checking my emails to see if octopoid_horror has made his next move yet. At one point we had two games going at once.
Best of all, Steam has it for £15 for two copies - so you can give a copy to a friend (or do what I did and go halved with a sibling you like to shoot).
Best of best of all, it can save off games to YouTube, so that you can see my one victory against octopoid_horror so far*:
The actual game is sharper than that, obviously. And as the video lasts 38 seconds and starts at five seconds, you can tell it lasted 7 rounds.
The turn based thing is particularly awesome, because it means that I can have games with people without worrying about whether I can be somewhere at a particular time. So I can actually play with other people! If anyone fancies a game, you'll find me as AndrewDucker, as per usual. They're consolidating servers at the moment, but until they finish playing with their infrastructure you'll have to use the uk1 server to start a game with me.
*He's won one, I've won one, we have one draw. The battle will continue tomorrow...
Original post on Dreamwidth - there are comments there.