I'm still using a DS Lite and I never felt the urge to upgrade to a DSi due to the scarcity of "good" titles. DSiEnhanced games didn't really bring much to the "plate" either, and the only reason for me to get one would be DSiWare and the "Collector Value".
I don't think I'll be getting one anytime soon though, I'll wait till they're cheapo and it will most likely end up on my Console Trophy Wall, never to be touched (at least not much).
I feel pretty much the same about the 3DS since as of now it's expensive and there aren't all that many good titles for it just yet. The 3DS WILL have a good library though, so it's a different story.
Truth to be told though, I program for the DS more than I actually "play" on it - I finished pretty much all the games I was interested in so-far and right now I'm occupied with the PSP. I'm pretty sure I will find some more "gems" for the DS, but until that time comes, it will remain a piece of hardware that I play with rather than play games on.