Pros
Practically Nothing. You meet a few colleagues who share the same dissatisfaction to the company. That is all.
Cons
Everything else. From management to the hours, last minute change of location (it really is last minute, as in you'd get to the agreed location then be told to go somewhere else). You're Job is not guaranteed. You have to meet ridiculous targets. 2/3 hours of unreal training (which is supposed to be paid for but they don't pay you for it). If you dont meet a certain target by midday your dismissal is on the horizon. You'll be dismissed by someone else, not the management team, you have no chance to appeal your dismissal (not that you'd want to). Management aren't professional at all. They don't care about quality or etiquette, it's all about numbers. Your constantly nagged about signing up more people so instead of being genuine and getting people who are really interested, you have to play on people's emotions. I could go on for several pages but if you wish to do charity work, work directly through a charity, not some cheap third party organization.