The Washington Post has a very interesting article questioning the validity of various games' claims that they can enhance your mental ability. People are suckers for quick and easy ways to self-improve without putting in any effort, and games like Brain Training and Big Brain Academy are there to give the lazy masses some of the mental stimulus they crave.
It's a pretty interesting read, and seeks to compare the marketing claims of these cognitive training games to the actual benefits they may provide. One worthy point is the argument that while brain fitness titles might improve various isolated aspects of the mind, that's not necessarily an improvement of the gray matter as a whole:
I'm a natural cynic, so the only interaction I ever have with Brain Training is when I look over a DS player's shoulder on the train, see that's what they're playing and then sneer with a smug and elitist contempt. I'm doubtful that they can do much more than make you better at the puzzles contained within the "game" itself, just like playing Street Fighter a lot makes you better at pulling off Hadoukens and Command & Conquer makes you better at knowing when to build the Barracks. In other words -- while there is certainly educational merit in videogames, it's unlikely that Brain Training will get you into Harvard, moron!