First off, this is a very accessible game and is a blast to play. You just click to attack and dash/jump/kick. The environments and enemies are well-designed. The boss battles are the highlight of the game, but they can be a pain.
Like all good boss battles, there are certain patterns that you need to figure out rather than just button-mashing. However, the game actually punishes you for not getting it right away. You could be doing well in the level, fail to figure out the boss pattern quick enough, die, and if you "continue" in any way, it reloads from the nearest checkpoint, rendering it nearly impossible to avoid a "D" grade. While it makes sense for the score to reset after death, there should also be the option to restart a level from the beginning for players to "redeem themselves" in that particular level. You could just do a second playthrough, but the sole incentive of beating high scores isn't enough motivation imo to play through this game more than once. Might be different if this game had achievements however.
Apart from this minor complaint, there is little else wrong with the game. I would recommend it for everything it does well -- the fun of beating things up, the music, graphics, and the barely comprehensible but awesome sounding story. If you enjoyed the demo, then you wouldn't be doing yourself a disservice to buy this game.