The game is fun to play and the core mechanic is simple and easy to understand.
After a while the loop starts to feel a little repetitive, so a bit more variety between sections could help keep things fresh. I did like that each section ends with a boss fight, though, and that the bosses vary in their attack patterns and difficulty.
I also really liked the way progression is shown through the portraits of the defeated bosses filling with color in the main menu. It's a small detail, but it gives a nice sense of progress throughout the game.
Overall, a very good and enjoyable game!