Be aware, all, that these games do promote cognitive growth -
but only in the game and extremely similar processes!
That is to say, playing these games will not increase your productivity of studying, your ability to comprehend books, or your mastering of mathematics!
They will keep certain areas of the brain "running", and keep them from deteriorating, but if you weren't using them in the first place... maybe they should go away?
The best brain health is to learn many small paradigms, many small perceptual shifts. Learn things you don't usually get into, like perhaps economics, or the reproductive cycle of some plant or animal, the atmospheric composition of jupiter, the trends in social or demographic change in cities, etc.
Learning many small things in many areas creates great amounts of synaptogenesis, but learning brand new things creates neurogenesis - that ultimate goal of the genius trait
Hope this helps. Another way to say this is, once you have mastered any of those games on that site - move to another one. This demand for new learning of many different things tells your brain "More power is needed!" and instead of "trimming" and making efficient old processes, the brain will turn its focus on bringing in new knowledge!
I LOVE LEARNING.