My 4 year old has a Leapster. She got it for Xmas last year, she was only 3 years old when she got it. She LOVES it. They now have a newer model of it called the L2...the screen looks smaller to me, but supposedly there are some neat features and some of the newer games coming out can be used on both-except that there are more features to the games when played on the L2 instead of the original.
My daughter needed a little bit of help at first, but she caught on fast. It is something she chooses to play with almost every day. She really enjoys it. Here and there she needed help with some of the concepts, but mostly she can do stuff on her own now. It helps them learn letters, sounds, numbers, etc. Rachel can now do VERY simple addition because of one of the games she has!
The way I saw it...even if she wasn't ready for it at the time that we got it for her-it was something she could 'grow into'.
Good luck.
A little bit of advice...if you can, pick up the adapter, because it eats batteries like mad. And watch for sales on the games...once in a while they will clearance some of the titles at Wal-Mart. And if you can, buy the extended warranty. We bought the Leapster at Toys R Us and bought the added warranty so if it was dropped or broken in ANY way, it would be covered.