I'm having to take breaks from it but I do keep coming back and clearing things each time. That said, I'm not very far in the game yet (I don't think). Bosses have an option of "simple" or "regular" difficulty and I wouldn't recommend "simple" because it's not any easier. It seems they just remove certain phases and it locks you off from content later. As one reviewer put it, "your choice is between really fucking hard and excruciatingly difficult with some reward for the pain".
Twice I have scraped my way to what I think is the end of a boss only for it to transform into something even more difficult. I've literally shouted NOOOOOOOOOO at my screen both times. I suspect this will continue to happen.
I think if you have any nostalgia for this style of animation then the game is a no brainer (to get) - I personally don't because I always associated that animation style as a child with "oh no I'm about to get a history lesson from X family member and I just want to watch cartoons" - I also found the musical style jarring. But even considering my feelings toward it, it's an absolutely beautiful game and I've been bopping along to some of the tracks and thoroughly enjoying myself.