My DD5 loves her Leapster. Between me and the GPs she has collected about 8 cartridges and loves them all. There are a lot of cartridges for Pre-School-K: Dora, Nemo, Backyardigans, Letter Factory and Princess are great beginning cartridges. The handheld has a few games that are also fun.
I love the Leapster. We let her play it for a short while each day (usually on the potty) plus in the car (airplane etc). When she first got it last Christmas, it really helped her with her letters. She's moved up to the more advanced games and is loving the math-based games, but still loves playing the "younger" cartridges, too.
I say Leapster is worth the investment.