A lot of comments here are focusing on the pricing and ignoring the 'sandbox' part of the game's description. There is a leaderboard you can try to climb but it is reset monthly and isn't really shoved in your face. Beyond that you're free to set whatever goals you want and go about them however you wish, with the principles of other strategy games (harvest resources, build things, attack enemies, etc.) to give you ideas. It's really a game about you and your code and how it grows and evolves as your goals and strategies become more complex.

Screeps is definitely worth trying, if for no other reason than to learn javascript at a deeper level if you don't use it extensively. I do have a couple of issues though:

The first is that it is quite resource intensive considering its simple 2D visuals, both in the browser and the standalone client. I'm not sure if it is just poor optimization or the javascript runtime but it shouldn't make my machine hot. It'd be nice if this were addressed.

The second issue I have is the workflow, which doesn't lend itself well to source control. The ingame editor works well enough, but its idea of a "branch" is just a named copy of the code directory. I think there is a way to integrate with github but I don't want to upload everything to the cloud just to be able to track changes. It would be nice if this area were looked at and made more flexible so we could use our tools of choice.