For $13, I'd say that so far I've gotten my money's worth. The game is in Alpha, and is receiving updates at a decent pace as well. As I've played it, there have been a few things that I feel would make it better have they been added, but the game's still Alpha, so long as they keep working on it, I can only see it getting better.
-Excellent music (one of the main things that brings me back to playing it at times)
-Good variety of weapons along with tiered progression
-Decent variety of survivors to find, and an okay amount to pick from to start off the game with
-Plays very well on my laptop, minimal lag and such, so it's a solid experience that isn't interrupted by FPS drops
-It can get repetitive after a while of playing
-Entirely Single Player at the moment, no idea if they plan on some kind of Co-op or something
-Obviously Alpha, so there may be a few bugs here and there (many of which in the past have been successfully patched)
Well that's about all I can think of off the top of my head at the moment. It's a pretty fun game with excellent music and it's an enjoyable way to spend a bit of time. As of this moment, I'd say it deserves an 7-8/10, it's fun, but breaks should be taken every so often to not tire it out (like many other games of course), and it's still in Alpha so we don't necessarily know exactly where it's going from here. (Also I might update this every so often as the game progresses, maybe I won't, we'll see).