This game is just fantastic. The lighting, soundtrack and delectable graphical style combine to create a wondrous atmosphere which manages to evoke both feelings of sheer amazement (whenever you come across a truly beautiful cavern, for instance) and of adrenaline-pumping tension (trying to survive through a swarm with barely any ammo left will cause that).
I've had a lot of fun with this game and I look forward to much more, considering that the developers are some of the best I've seen yet: we're talking about devs who openly speak to their playerbase and consistently update it with new mechanics and content every one or two months (they do postpone sometimes but it's only for at most a month, always for good reason and they always make up for it).
This is obviously a coop game, but the developers are truly putting effort into making it fun to play alone too.
I could go on and on about how much I love this game, but I think I've said enough.
I'll wait for this game to get out of early access before I give it a proper rating, but for now I can say that it's on its way to a full 10/10. I couldn't fathom any way for it to be better.

Rock and stone, brothers.