I've never been a super-gaming type person, generally being more interested in doing something than playing video games. I'm also have a personal tendency towards addictive or compulsive behavior that makes me try and steer away from getting to carried away with fun stuff which doesn't have an obvious redeeming value (Steven Johnson disputes the idea that video games are total time wasters in his book "Everything Bad is Good For You"). He has some points - but of course moderation and balance would have to be taken into account no matter how fantastic a video game is at sharpening up your bean.
I've written about Brain Age
I've also become rather fond of Animal Crossing
The IGN.com article The Science of Your Brain on Games that tells me a professor Gee, or maybe Stephen Johnson, thinks Katamari Damacy is one of the "best brain games" (the article is a little confused about who said that...maybe they both did...or neither one of them). I'm not real sure if IGN - "a unit of Fox Interactive Media, Inc., a leading Internet media and services provider focused on the videogame and entertainment enthusiast markets" is the most unbiased source for information on the benefits of playing video games.
I'm amazed if anyone said Katamari Damacy was a good "brain game". I've played it for maybe 30 hours and it seems more like a good "thumb game". You use the two joysticks to move the ball forward/back left/right to pick up bigger and bigger objects - it's fun and sometimes funny (and a little violent since you can role up cats, and cows and people as the ball gets bigger)...but I'm at a loss to see what the brain exercise part of it is.