Yeah I've got one. They aren't all that great, but at the same time they aren't nearly as terrible as sega purists make them out to be. The sound is different, but not horrible. The model with the wireless gamepads is garbage as they are using IR to transmit the data (like your tv remote) so they don't work unless you are pointing the gamepad right at the unit. Also flash carts and oddball "special" carts won't run on it. That being said, you get 40 decent games (not 80... 40 of them are homebrew junk) for 30 bucks which isn't a bad deal.
The thing is... no rgb out, so it isn't going to look that great on a modern tv. Heck you don't even get stereo sound. These things retail for around 30 bucks and a real genesis can be had for the same price. So get a genesis, spend an extra 40 or so and get a flash cart for it.
It's exactly the same deal with the atgames Atari and other similar units. They are ok, but generally not up to the standards that us gaming nuts expect.
The reason people are flipping over the NES mini is the hdmi out. Since it takes costly, extreme, modifications to get rgb out of a nes this is a big deal.