Good but too easy
The problem is that the later worlds are not nearly hard enough. I am not a great gamer but I managed to complete the game very quickly. Only died a couple of times throughout. Plus, koopas at the end can just be killed with fire flowers. If you store a fire flower, you just activate it when the koopa appears and just kill them off. Even the last one.
Also, two worlds have to be unlocked, meaning that you get the the final kooper even more quickly.
In summary, fantastic game-play, graphics and ideas let down by being too easy and too short.