The gfx are cute, the story is good enough, but unfortunately the gameplay has some major flaws.
* Jumping upgrades will HURT you. They'll make it more difficult - or even impossible - to make some jumps, forcing you to take damage or to forfeit certain coin rooms. They'll also make it more difficult to hit flying enemies, like those in the last two levels.
Unfortunately, jump height isn't really controllable, and you cannot downgrade again. So I paid lots of coins to play hard mode?
* Coin grinding. Why oh why shall we repeat levels over and over again just to afford the upgrades required to beat the game? How do you benefit from wasting our time in such a way? It doesn't make the game more fun, it turns a nice little distraction into something way too long to enjoy.
Repeating once or twice is fine, but having to do it a couple dozen times just shouldn't be required.
* the pits of death and uneven difficulty curve have been mentioned by others. Unavoidable damage is always a bad thing.
* the different weapon types could have been unlockable by gold, and switchable in-level. Otherwise, you'd have to visit the level, die a horrible death because the piercing shot couldn't hit a single enemy, then go to the shop to switch. Not fun.
* I'm not sure the system map was needed. A single level-selection screen including the shop would have sufficed. Most star systems were almost empty anyway, containing only two planets. I also hated the delays in the menus, making them feel unresponsive and laggy.