The idea of this game is to clear levels by guiding clockwork mini marios around courses of increasing complexity. Completing the levels is relatively straight forward - you can stop and start the mini marios whenever you want - allowing you to stop and think a bit. But to get a good rating, you have to collect all the coins, get all the mini marios finishing at the same time (chaining), and what's more, do it quickly. A very different prospect! Fortunately you can unlock the next level with a poor rating.
There have been two main criticisms of this game. The first is that it is too easy. I'd disagree. There's over 80 levels and, as well as completing them, there's the added challenge of completing them with a gold rating. I've contented myself with getting a silver on all levels. That's hard enough! The second criticism is that it is repetitive. On that score I'd agree. But this is a game designed to be dipped into every now and then. The controls and rules (which are very intuitive) need no re-reading. You can pick it up whenever you want and just dive straight in.
It's not new or ground breaking. But when you throw in the level designer, you've got a well rounded game, finished with the usual Nintendo polish, that is what the DS was designed for.
Hope this review was useful.