Not So Fast
As others have pointed out Lieury's hypothesis and conclusions are sloppy. A broader ten week Scottish study last year came to the conclusion that the Nintendo games did improve math scores and by more than 50%. So take your pick.
On the other hand, Nintendo isn't the best out there by far. For a study on a more serious tool Susanne Jaeggi and Martin Buschkuehl's study on Improving Fluid Intelligence by Training Working Memory (PNAS April 2008) recorded increases in mental agility (fluid intelligence) and short term memory of more than 40% after 19 days of focused brain training.
I was so impressed that I contacted the research team and developed a software program using the same method so that anyone can achieve these improvements at home.