This is always one of those things that can go great, or blow up in the bands face. I remember one instance where it did work out.

Many years ago, I was at a festival in Florida that Avenged Sevenfold was playing. And Avenged Sevenfold would cover "Walk" from Pantera. But this time they asked a fan to come up on stage and sing. But M. Shadows was very adamant about whoever came up. You had to know the actual song, not just the "RE... SPECT... WALK" part.

So they got a fan up on stage and he proceeded to do pretty great. He wouldn't be fronting a Pantera cover band anytime soon, but still knew the lyrics and it wasn't a complete assault on your ears.

Now, just a couple days ago, Green Day brought a fan up on stage in Chicago to play "When I Come Around", and you have to give the fan credit, not a bad showing.