I own a good fourteen or fifteen.
A lot of them are crap games, as the good ones like Mario Kart DS, etc. I sold when I got my SuperCard SD.
I really wish I'd of kept on to them, for collection purposes (though I doubt any of them will be worth very much at all in the future, but just for keepsake or whatever).
Most DS games aren't worth the $30 dollars to be honest, but there's the odd one that's worth buying and I actually do buy them occasionally.