The amount of sudokus are worth the purchase...
That's because the game wakes you up. Some simple challenges each day actually seem to stimulate the brain.
It doesn't actually make you smarter, but it does stimulate you to think quicker. After getting the ideal brain-age, you are probably able to answer basic mathematic questions without having to think about it - memorize a bit more, concentrate a bit better.
But, if you want to become smarter - read some books, scientific journals, great classics, take a course in whatever you are interested in...
It does seem to work as a brain stimulant, and even if it didn't - it's just a fun game to play and challenge yourself on something other than the standard fare of puzzle-solving & timed reaction.
The mayor downpoint of this game is the sheer amount of clones it produced... the game industry does seem like star wars episode II at times, but it was never so apparant as when this game turned out to be hit.