If you get this game, get used to dying. A lot.
The game play is fun, and super addicting, but it's designed like a kaizo rom hack. There are only four worlds, with 3 levels plus a boss in each level, yet it still took me over six months of playing it an hour our so a day to finally beat it. It just took that much skill, testing different upgrades, and luck.
That doesn't detract from the game. This game is great for all those times where you only have 30 seconds - 1 minute to play, like waiting in line or for the train.
The graphics and music are also amazing for an indie game.