I stole a cousin's for a day this weekend. I was (happily) surprised when I turned on Metroid Hunters: First Hunt, and found that the cutscene movies were crisp in both sound and video, even nearly GC quality. The in-game graphics weren't quite as good, but still excellent.
Mario was fun too, and it was great playing as Yoshi. GBA games DO look even better on the DS screen, too.
Conclusion: After the PSP comes out and the price for the DS drops, I'm getting one.
I wish i could get the DS. I saw it firsthand and its looks pretty nifty. Didnt see any gaming action but it looks awesome. PSP is not going to overtake the handheld war considering the defects the PSP got in Japan.
Check it out http://www.gamespot.com/news/2004/12/17/news_6115293.html
In short, the pixels on the screen are dead, and if you move the PSP one way, the disc shoots out.