I saw one of these things at PAX. It’s a great idea; after all, the actual hardware involved has become extremely small. You can fit an NES into a cartridge if you want to. So it makes sense to stick all the circuitry in a box, add some cartridge interfaces, make some ambiguous controllers, and sell it as a super-console. The one I saw was NES and Genesis, like this one, but there’s a new one that will include SNES as well. The regular is about $40 and the other one I’m guessing would be $60-70.
The bad news is that the hardware isn’t original Famicom and Sega stuff; I was told by the Pink Godzilla people (or else read on the box, I don’t remember) that it doesn’t support all mappers. To the layperson, mappers are sort of the ground level of a game, governing how sprites move about, how game objects in general interact, and so on. Final Fantasy would run on a different mapper than 3-D Worldrunner, for instance, and tempting as the SG/FC is, I wouldn’t want a console that can play one but not the other.