I haven't done it, but I have 2 friends who did. Sadly, both of them say they would never do it again. They had to do a 4-hour shift and in return, received free admission to the show. One is in her mid-40's, the other in her mid-60's, and they both said 4 hours was too long. The younger one said she was so tired by the end of her shift she had no desire to see the rest of the show.
I might consider it if I could do 2 or 3 hours, but that's probably unlikely to happen because the show would need more volunteers to cover.
eta - This was at the largest show in Portland - a smaller show would probably be easier.