This game is an absolute masterpiece, even disregarding its art style it's gameplay is so simple but extremely effective, using it's fairly basic mechanics in creative ways it takes a bunch of basic ideas and perfects them beautifully. This game keeps you guessing and ecstatic to see what's next; every boss and enemy has its own quirks and unique attacks. This game gives me a feeling like no other with its endearing designs and terribly hard challenges and boss fights. The soundtrack of this game should go down in gaming history by mixing swing, jazz, big band, and ragtime in different areas to amplify the experience.
(I would highly recommend listening to Shootin n' Lootin or Aviary Action)
The amount of effort and love that exudes from this game is astounding, quickly becoming my favorite game ever. And when talking about the most criticized aspect of the game, its difficulty, I feel that one of the creators Jared Moldenhauer described it best "It's tough but fair"