Slow and frustrating start, but takes off once you learn what you're doing! (No Spoilers)
There is always a sense of progression and a strive towards it. You get to learn some tricks of the trade, and for the at least 20 hours I've played so far, not for one second did I stop learning something new. If there's one redeeming quality of the horrible tutorial, it's the bright light bulb feeling you get for figuring everything out yourself.
Battling the bosses is just like the rest of the game, frustrating at first but a lot of fun once you get the hang of it, and it offers a great bit of diversity of fighting and strategy compared to the other enemies.
Another strong point in the game is the the accessories you find underground, all of which are useful in their own way and can completely change the way you play the game and navigate underground!
My single biggest complaint is that at a certain point, and possibly more, how next to progress is not obvious, and figuring it out for yourself is near impossible, and I personally spent a lot of time on Google trying to figure out what to do next.
In summary: As a guy who had no interest in a game about mining, I was astonished by how much fun it can be to move up in the world by digging yourself deeper into a hole. And as the Classification says: I was so addicted to this game that I couldn't play the newer game I just bought.