I like the simple user interface and the graphics. There was a good amount of unique sprites to keep things varied and challenging as you progress, yet the sprites have a relatively simple design that it's not too distracting when attempting to memorize. The music is fairly ambient and not too distracting, though having an option to lower the volume or mute the music would be nice to have for focusing purposes.
I like the endless mode-type gameplay, as that encourages practicing your memorisation skills in order to reach the more difficult sections. The gradual increase in the quantity and types of sprites was a good choice. I think having a secondary mode where it's more test-based would complement the endless mode. For example, there are a set number of configurations and questions (though the question content and sprites used can still be randomised), and the mode only ends and gives a final score once the list of configurations and questions are exhausted instead . Such a mode would give players a chance to attempt at the harder configurations.
A challenging brain-training game.
P.s. I only got up to 12 - I should not have attempted this before going to bed!