I bought this game a few days ago and it hasn't been out of my DS since!! It is truly addictive, and if, like me, you are a competitive person, be prepared for some late nights as you try to beat your previous score, or to obtain as many A+ ratings as possible.
It starts with a brief challenge to see what percentage of your brain you are using, and you can then choose to either take more tests or do some training. There is plenty of variety in the types of puzzles that you have to solve, though in training mode, you have to unlock the games before you can practice. I haven't figured out how to unlock the games yet, though I think it is based on your brain level, amount of A+'s gained and the amount of times you have played the game.
If you are feeling very brave you can try the stress training or the stress test, where you have to solve puzzles while there are other distractions (for example you might have to put the answer in a particular box and if you get it wrong you get shouted at!!)
I have got a few of these type of games, but I have to say that this is the best by far. If you are thinking of buying a game in this genre, then get this one - you won't regret it!