It depends on the organization and their methods. I don't mind local Cub Scouts or Girl Scouts as the kids are always polite and never aggressive. Sometimes I buy or donate, sometimes not.
I HATE the bell ringers, not because I don't think they are a good organzation, but because the bell ringing is so annoying. While I might donate to those who are just standing there, I NEVER donate to the bell ringers who ring their bells loudly and continuously in front of a store. I feel so sorry for the people working at the checkouts who have to listen to it all day.
I agree with others here that when you are confronted by the same organization every place you shop, it gets to be way too much.