For ages I've been waiting to buy myself a GBA SP, but I wouldn't do it if I couldn't buy a flash cart along with it. Then some magical money appeared out from nowhere (no asking!) a few weeks ago and I was like "why not?", and got myself a DS and a 256MB flash cart.
I won't even comment on the GBA part, since it's been awesome, moreso when I can play SMS, NES and even GB/GBC games with the flash cart.
On the DS side, I only have the Metroid demo (waiting for extra cash to buy myself a DS game), but so far it has been very, very good.
Screenshots and even those streaming vids on gaming sites do the system no justice: it looks and moves super smooth on the real thing, and the lack of bilinear filtering is almost unoticeable, and far beyond PSOne graphics: no texture warping and no jittering at all.
I suspect the system runs at 24-bit color depth, because the color gradients are super smooth, with no ugly 16-bit-ish color banding nor dithering (thanks God, because dithering looks horrible on sharp LCD screens).
Ah, and the touch screen input in Metroid was very good, almost as responsive as a keyboard+mouse combo.
It's also far more intuitive for people who barely played any FPS games because both my two younger sisters (8 and 11) managed to play the Metroid demo without any major problems (the jumps are harder for the younger one), and they actually *enjoy* playing it. Same goes for my non-gamer younger brother who can't play Doom for more than 5 minutes. I got him playing the Metroid demo when I wasn't looking once in a while (well, that before he found WarioWare in the flash cart...).
I gave my sisters a rough explanation ("touch and drag on the touch screen to look around, press here to shoot"), and they were running around killing monsters in no time. They also didn't get too confused or totally lost, like when I tried to get them to play Outtrigger. They also fiddle with the bios menus and the pictochat thing with ease.