I gotta say I got really addicted to it. I spent hours of a Sunday clicking oftenly as I did other activities and left the game open.
I'd say the beginning is not as fun as when you open the shop that allows you to get other metals, as it starts veeeeery slowly. I had to click like mad until getting metals and ores started. But it managed to always keep me hooked waiting for the next small upgrade (though it ain't very hard to lure me into such I admit). Changing the terrain by depths took way too long, granted, I got gold like mad once it changed, but maybe more terrain changes with more gradual improvements would feel better.
I can do other things while playing the game, as I was programming lightly and at night I played while watching an eSports tournament!
I'm not sure if it was a bug in my browser but the game had no sound. While that worked well when I was doing other activities, if you are only playing the game it gets a more chore-y feel. Also, when I get the inventory, I feel slightly lost because I have no idea which material is which, maybe adding their names right under each item even when you have none could help.
One glitch I noticed was that when I'm looking at the inventory, clicks are still valid in the game's buttons. So you could be holding right button, but if you click left button you'd just spend resources on something you are not aware. A minor thing that barely changes the game at all, but could be helpful, is that when you have a window open, like the Mine. Its tooltips did not update when you get the needed materials for the upgrade. It really is no big deal though, you can do the upgrade just fine, it would just look nice to see the text in the tooltip turning white.
Congrats on achieving a rather addictive progression!