i own the madcatz one and i truly love it. it's hard plastic and insulated very nicely. you couldn't damage your gba-sp if you tried. only downside is, it adds significant size to the system, so you won't really be able to fit it in your back pocket anymore. it also lets you hold 3 games in the same case. you can get it solo for like $10, or with a headphone adaptor, cheap headphones, and a car charger for $15. (at least those were the launch prices; they may have dropped since).
if you're looking for a smaller option, i remember reading about something called the rhinoarmor on pgc which is a very thin metal casing that fits around your gba-sp. it still allows the system to retain it's complete functionality, so you leave the casing on all the time, even when playing games. i haven't seen it for sale anywhere except the manufacture's website, but you may want to search around.