My two boys, ages 5 and almost 4, but have one. They received them over a year ago for Christmas. At the time they were the best - we got them because we were going on vacation and thought they'd help on the plane ride. They are great - educational games that seem to keep the kids amused. I do occasionally hook them up to my computer because they also like Leap World online. They both have Nintendo ds' now and would prefer to use those but the games for that aren't really educational but they don't argue when I say they can use the leapsters instead. I don't know that there are any cons to them in my opinion. We are still getting plenty of use out of them....maybe they are a little bulky... I don't know about the new Leapster Explorer but it might be worth looking into.
They are great my son has one and he is 5,, he has had it since he was 3 i think, and its great. Problem is,, more kids want the DS now cause leapster is for "kids" but now the DS does have educational games not just regular games. I'd start them on Leapster and go from there.