I hate them. They're so aggressive and do the charities no favours. They get paid commission per every sign up as well and I remember reading somewhere that it would take something like 18 months for your direct debit to actually go towards the charity after paying off the chugging company.
I usually always have my headphones in when I walk around anyway so I just walk past them ignoring them, even if they wave at me etc.
I even had one at my door the other week
