I have a RetroDuo 2 (Red and Black), and I love it. Thus far, I haven't run into a game that doesn't work, and I got the RetroDuo because it is one of the few famiclones that can play Castlevania 3 for the NES and Star Fox for the SNES.
Only problem is that it only accepts SNES controllers, which means no light gun action. Still, that's a pretty small price to pay.
Plus, it runs Super Game Boy perfectly, so it's almost a 3-in-1 device. Love playing some Mega Man V and Link's Awakening.