It seems like a more reasonable way to control than the four-way pad + four button arrangement prevalent on most PDAs and cell phones (although ideally I would prefer a tilt-sensor for a virtual-cockpit arrangement, with a button to press down whenever you want to look around so every little jolt doesn't pull a Linda Blair.) Alrthough, really, pretty much anything is better than the former - even the typical elementary NES gamepad isn't that well suited to this sort of thing; Prophecy GBA's controls were *okay*, but even ignoring the macros needed to pull the more advanced features there's still room for improvement. For one, the eight-way direction pad isn't a real mini-joystick; it only goes direction yes/no, with no way to pull a hard jink from one side to the other, for instance.
The iPhone-type multi-touch-screen idea or such has an idea here with the way finger-touch velocity affects movement, but I don't want to have to block out the screen whenever I want to move. I've heard of concepts placing touchpads on the *back* of devices, so the screen isn't obscured for this sort of thing - that could be promising.
That may be true -- I don't think I've played a DS game yet where the touch screen wasn't an afterthought (use it to scroll through the Pokedex!) or some kind of obvious gimmick (Hannah Montana has to find five pixels on this screen, use this magnifying glass to help!)
I wonder what the perception is for the DS Zelda game. I have not played it myself yet, but apparently the only way to control Link is with the touch-pad, so it might not exhibit quite the same kind of after thought feel.
(Also, Star Fox utilizes it for control as well...)
In Metroid, the touchscreen actually IS kinda useful... but I don't like it. In the Zelda game, it's better as far as controls go, since they make it actually useful. However, as a rule, I generally try to stick with the normal controls when possible as it's easier to hold a DS with both of them.