Its hard when they're for charity. the people who are organising generally dont have a clue.
You just turn up and play. your not getting paid so if you go with an open mind as WoT says you never get too let down.
put it this way, you played a few numbers, probably had a few pints and at least now you know the guys a d**k.
As someones already said, don't let it put you off as some of my most enjoyable gigs have been for good causes and you're raising money for something worthwhile.