I actually just finished this game today. Overall it was pretty amazing. Art style, music, and level design were absolutely perfect. Loved the themes it explored, too.
My main issue with the game - my only issue, really - is that the levels, and especially the boss levels, are way too long. I was playing the final level for like three hours. That's kind of ridiculous, IMO. (To be fair, I stopped playing the game for like six months and then came back to play the last level, and thus I was probably dying quite a bit more than I would have if I had kept playing the game until the end.) And every time I was doing a boss level, I just kept thinking, "WHEN IS THIS GOING TO END."