Except for how it bloats the GBASP, that is awesome.
Actually, it'd be funny if it worked with NES cartridges... you'd see this huge cart bigger than the GBA sticking out :)
The MagicBox states that there'll also be an adaptor for, er, the adaptor, to allow it to play NES games. If the video output is clean like GBA then this'll be great. If it's like NES TV video, then we'll just have to keep waiting for an RGB enabled NES clone.