These ones?

Here's an amazon post for one (that's no longer available) but it has a several answered questions and reviews. This video is a review/breakdown of it, almost 30 minutes, with some gameplay examples and such.

That's enough info to gist, at least. It runs any Genesis game except Virtua Racing. And it can emulate Master System games on your Genesis as well. (I don't know if that's on the card or through software, though.)

Seems to be fine, as a product in general. But like I said, it's still second-hand electronics.