I've been wondering the same thing. Though everytime I feel like buying one, a new one is announced and the other ones don't drop in price.
I'm probably just going to repair the one I got (screen fell off) and use that one.
If you do get a DS then don't get a 3DS, as that one is screaming for an update. In 9 months, one will probably release that actually has batteries.
The biggest problem with the DS I can see is it has so many crappy games and shovelware, that I'm having difficulty finding the good games for it. Pokemon is a no brainer along with Mario kart I guess, but What else? Castlevania Dawn of Sorrow (which NOWHERE CARRIES ANYMORE) and Metroid Prime Hunters? I'd love suggestions.