I've plugged an acoustic guitar into the mic input successfully. I've never plugged in an electric guitar, like a Fender or Gibson. Electric guitar amplification is an entirely different kettle of fish, which can be done through the Genos mic input but it won't have the same sound as a Fender amp, despite the fact there are numerous electric guitar effects on the Genos. An electric guitar amp and speaker combo is skewed very differently than the flat frequency response produced by the Genos sound processor.
Edit
I agree with Mark in that it will produce sound, but don't expect the same results as a real electric/amp combo, if that's your goal - not even close.