Bottom line, for NES and below it works alright. If you can find one cheap, say $20 or less, it makes a nice toy. I can’t really recommend it and I would never consider it a ‘go to’ device.
NES: OK most of the time. Games with lots of sprites on the screen will cause slow downs.
Game Boy/Game Boy Color: Game Boy games work very well. Game Boy Color games suffer from the same problems as NES games; slow downs at inopportune times.
SNES and other 16-bit consoles: Too slow to be playable.
The speakers are not great. Everything sounds tinny.