We've been talking about Sonic Jewels 7 for awhile now. The SEGA Master System and Game Gear emulator fetches for $1.29 in the Marketplace and features the ability to load ROMs via URL or Skydrive. The emulator itself is exceptionally well designed but it was lacking one major feature: sound.
Now with the latest upcoming update (version 1.2) the dev has completed the sound-emulation aspect (no easy task) and we've managed to get an early look.
So far it has performed very well for us and our HTC TITAN (1.5GHz CPU). We have been able to max out the frame rate with sound and have seen no degradation in performance. Older devices may have some issues though which is why they can "skip" frames via the settings to help smooth out the graphics during gameplay.
Overall, the sound emulation really completes this experience and we look forward to the updated version hitting the Marketplace soon. For now, you can use the no-sound version here with a trial.
[And to fend off criticism on our earlier editorial, we want make it clear we're not against emulators per se on WPCentral, just against people selling other people's software which is what Rivera was talking about here]