It's free people. Get it while it lasts (maybe forever)!
You'll need Steam to play obviously, so far I played it on easy and it's pretty fun, but ammo runs out really fast, and parasites are annoying as hell. There's only 1 campaign divided into like 7-8 chapters.
I also downloaded the SDK. Apparently you can generate maps in-game as tiles (with a tool called Tilegen). It'll be interesting to see what will come out of this. Hopefully I can make a map worth playing before 2020.
Hook me up on Steam - penpen35 is the name. I suppose a few more guys would be getting as well. It'll be fun.