My two girls, ages 4 and 5, play their Leapsters non-stop. In fact, we had a buy a second one for my youngest because she kept taking her sister's under the kitchen table to play it.
Wish there were more games, but so far, the kids have loved every game they have tried.
Instead of a gameboy, we chose to go the Leapster route for our 5 yr old son when he was then 4. He didn't touch it for about a year. When he turned 5 and was in Kindergarten learning his ABC's, that's when he really took to it.
I wish they had a wider selection of games to choose from, so from that standpoint this item is somewhat still limited. If you're in Canada like me, you'll be frustrated with even less choice that we have unless you order it online.
A great learning tool, something I don't mind my son picking up anytime he likes!