I have had many good expieriences with Used things, my Ds, the clunker original Ds, is still in great shape and works perfectly and I bought it used. When I bought my Pokemon Platinum used It came with many EPIC pokemanz like Arceus, Darkrai, Mew, Deoxys, etc.as for DSi, its a brick, no GBA slot so it fails at life and magicaly transforms into a brick.
the dsi is a awesome upgrade from the ds and ds light. the fact that you can download games is enough to make it better then the original. it is basically a portable wii. if you want to play gba games then get a gba or keep the original ds or ds light after you get a dsi.
its not that great.the touch screen doesn't even use touch pressure any more.. so no matter how hard you press it won't make a difference..a camera is cool, but at the cost of gba slot dsi is fail.the only thing pleasing about it is the way the menu looks, and a microSD slot.Nintendo also thought they could block flashcarts with their 1.4 firmware but they fail hard.