The nintendo demo vision is an interesting piece of hardware. Used with a special gameboy that has a cable run from points on the cartridge connector, it is somehow able to translate the eight pins into upscanned and upscaled composite video with mono sound. The quality is remarkable and is far better than super gameboy. No borders, just full screen gameboy on any size TV.
Some of the units are configured in dual gameboy configurations. From what I have learned, they are generally used to avoid eye strain while testing games, and were also been used at trade shows. Feaures of some models are: timer until reset, sometimes multiple video out for video wall, gameboy advance and color models. I've run into several different types, and will have a more detailed article in the future.