Did this with Shelter for a couple of months but quit today because I can do other, nicer work for the same money. I didn't dislike it, and was good enough to do it all Summer I think, (obviously it's basically a sales job, with a VERY high turnover of staff) but it got a bit tiresome being told to fuck off a couple of times a day.
Last Summer I worked for Gift Fundraising, an agency who get contracts with charities and basically keep all their employees in the dark about the fact they receive £140 for every donor signed up, meaning each person has to give for 18 months before the charity sees any money. Obviously if I'd been told this was the cost I wouldn't have taken the job and I get a bit depressed when I think how much I got paid and the demographic of people I signed up/low likelihood that they're still giving.
So yeah, has anyone signed up to a charity direct debit on the street or do they make your lunch hour a gauntlet? Has anyone else tried doing it? They pretty much hire anyone and it pays well so it's good for temporary employment if you're thick skinned I guess.