I quite enjoyed this, I have only spent about 20 minutes in game so far but I would love to find out how deep it goes! so I will be playing again later.
Music is nice, its quite subtle but its quite mysterious it does add to the vibe of the game.
And wow that skill tree, you have crammed a lot of stuff into this. Awesome work.
Only issue I come into (not game breaking) was if you load the upgrade tree > pickaxes, then select sell and then close pickaxe menu. You have then the upgrade tree overlaid on top of the Sell UI. I found just pressing 'Sell' again fixes this pretty easily.
Overall I liked it!