Not great, to be honest. Juggling save files is a chore, one that I never really learned how to deal with - the thing would just decide sometimes that I didn't have enough space to save the game normally or with a save state (this is what caused me to buy an alternative, ultimately). Pokemon Crystal and Puzzle League don't run properly, if at all. NES seems to run okay, but it's squished because the resolution isn't right as I'm sure you're aware, so I've barely used it.
I highly recommend either the Everdrive GB X7 or the EZ Flash Jr, both of which I've used, both of which perform MUCH better than the GB/GBC emulation.
I highly recommend either the Everdrive GB X7 or the EZ Flash Jr, both of which I've used, both of which perform MUCH better than the GB/GBC emulation.