The game does keep track of your score, so you're going to want to perform well, and it's frustrating when you don't because you didn't quite grasp the task. Replaying a Challenge is allowed, but it's just to make you feel better as it won't affect your score.
One of the more annoying things about the game is its utter insistence on your tapping things on the bottom screen. Yes, the DS (and 3DS) have the tapping thing as a big part of the system, but when not actively taking part in gameplay oh-so-many games allow you to use the D-pad and buttons to navigate the bottom screen… not Mind Gym, you need to have the stylus out if you're going to use the title.
In the end (and I know it feels like we just began), Puzzler Mind Gym 3D is just another in a long line of brain training devices. I am not quite sure why anyone felt they had to go out and make a 3D mind-training game (and the 3D used here isn't spectacular), but here one is. If you like mind-training stuff and your copy of Brain Age is worn down to the electrical board equivalent of a nub—or you're just looking to expand your brain training choices—you could do worse than Puzzler Mind Gym 3D, but you could also do better.
There is nothing terrible about it, but nothing all that great either. Frankly, unless you train your brain quite specifically to play every day, you may just forget about it completely.