I love the art and design of the bosses. My favorites are the sloth/laziness and greed.
I agree with Was A Horse about the traditional health bar, even if it is two or three extra hits you can survive. The black and white colour of the projectiles was enough of a hint for me to try and eventually get the hang of the exchange mechanic, but I think some kind of tutorial would be helpful.
I tried all the bosses, and had the best time with anger. He was difficult, but I found the other bosses much harder, but if I trained the exchanging mechanic more, maybe I would have a better chance.
Thanks for the experience =]