I had a chance to play with a DS for about an hour today. Size-wise it's quite a bit larger than the Gameboy SP in all dimensions, it has a decent heft to it's weight. It has 2 slots, one for older Gameboy games, and another for the new style cartridges. The battery is built in, and they brought back the built in headphone jack (no adapter required). Controls consist of a D-pad, 4 buttons, and 2 shoulder buttons. The screens look quite good, but the viewing angle is minimal, people on the side of you wont be able to see anything (which could be a good thing). I wasn't able to see the screen in direct sunlight, however inside it looks great!
I was only able to play Metroid Prime Hunters, but it looks great. The game action was fast and the graphics are exceptional for a portable. No slow down at all even with a screen full of enemies. The main screen is the action screen, the secondary screen gives you a realtime map. The controls were a bit clunky, controlling a 3D game with a D-pad and 4 buttons takes a bit of getting used to, basically you use one thumb to move, and the other to look around, while your index fingers work the trigger buttons for weapons. You can also opt to use the touch screen to look around, I tried this but it was very difficult to use.
I liked what I saw, but I want to play the PSP before I decide on a new portable (if I get one at all).