This game is fun; it really is. The time required to play is enormous for an 'idle' game, though. Each mine has thirty levels plus the elevator and warehouse. There are five main islands with five mines each, events that take multiple days for all but very seasoned players, and then Impossible Island, which takes 25 days just to unlock one mine. Mainland is far worse: There are over 120 mines of increasing difficulty, and I got tired of it after nine. In contrast, AdVenture Capitalist has three areas total.
All these games are supported by ads, but this game's ad load is next-level. Six ads per mine per day for double profits, two ads per hour per mine for barrier removal, ads all day long for increasing idle cash. Even with all mines on the same island eventually being grouped together, I figure it's around 50 ads per day just to keep the game moving. That's 30 minutes of just sitting there and watching ads -- or doing something minimal while they play. I'd pay $10 for an ad-free package.
If you're going to play, get the permanent double boosts or the game will crawl, same with any other idler. Otherwise, just be prepared to have it up and running all day.