In a few hours of testing, I haven’t seen any noticeable problems with the graphic or sound recreation on the NES Classic Edition. Even the flickering and slowdown issues that were a forced part of that original NES game design seem to be captured accurately.
Colors are rendered brightly and accurately (unlike similar NES emulation on the Wii, Wii U, and 3DS), with big, sharp pixels by default. So far, I’m really enjoying the CRT filter, which adds a pleasant fuzziness to the edges of the sprites without being distracting.
Sounds like a winner. Too bad the device doesn’t include the ability to install additional games.