My experience as audience at a weekly open mic is similar to Will Fly's above. As encouragement, I can say that I've watched several people grow from being nervous first-time public performers to confident ones who now get paid gigs. The styles vary, not all to my taste, but I'm impressed how many young peole are accomplished performers on their instrument. The standing joke, if it loks as if the line-up is a bit thin, is:"Roger, have you got your kazoo?" (does the Pope crap in the Vatican?) which usually stimulates someone else to get up and fill the slot! It is for young talented performers to gain confidence rather than ancient and too confident no-talents like me to empty the room!
It does fill the pub on a dead night.

Roger