I thought that the graphics were pretty good, I liked the sprite designs as well as the bosses. Also the backgrounds, they didn't distract from the level and they felt like a nice touch. The bosses themselves got tougher as the game went on so it felt like it had a good difficulty curve.
The music and sound effects were all good. In particular, the Cliff Crossing music was very good because it fit the stage well. It made it feel like you were on dangerous cliffs over the sea.
The game itself isn't too long, but that's good because it seems like it's more for passing the time with. As for the grinding, I didn't mind it, I tended to do it as I went rather than leaving all of it until the end and the secrets were good because they prevented the grinding from being too much. They were nicely placed so they weren't too hard to find, but they weren't in plain sight so it gives you a good reason to play the level again if you miss them. Having stats helped you to find them too because you'll know when you've missed a hidden chest.
My only complaint was that I didn't want to waste money so I didn't use the bombs. It felt like I missed out on using the shops a bit because of the 999 kupons achievement.
Overall, it was a very good game and I enjoyed it a lot.