I have the Genesis Flashback and oh the emulation is horrible. The sounds on Sonic the Hedgehog are just ear splitting!
I never had a Genesis as a kid so I missed out on some really good games back then, but the SNES was enough for me at the time. Later on I did discover just how great games like Streets of Rage 2 and Gaiares actually were.
Speaking of Zelda clones, I played one game on a Genesis emulator back in college called Crusade of Centy. That one was pretty neat. Looked and played a LOT like Link to the Past.