I saw this game on 1Ted59's stream earlier, and ended up playing with him. It really is a blast to play with people on MP.
This isn't a perfect game. It lacks things I would expect, such as a settings menu (eg. to enable/toggle fullscreen), private lobbies (or at least a way to select servers), and an offline mode. Also, I think the gates not resetting when using the rewind feature is a bug. But this is just too much fun playing with friends for that to really matter. Plus, I can't really complain at its price :P
Side note: I noticed there's windows and linux builds available for download. Any interest in making a macOS build? If you need a tester, I could try the game on my mac laptop.
EDIT: Turns out the ESC menu was broken when I played, and msot of what I wanted was already in the game...