They got me to sign up for one telling me how easy it was to get these giant scholarships for school. Turned out the giant scholarship was only $400 and not that simple to obtain and yearly union fees were around $350. So I called and cancelled my membership and they got rid of my benefits but were still taking money out of my paycheck. So I called them again and they said I would have to write them a letter and send it in a 3 week window around the time of the anniversary of when I signed up and I still had 3 months to go so I just ended up losing $350 for nothing. It was harder to get out of than a gang. Bastards.
Do you mean trade unions? If you do, then they do serve a useful purpose, but like everything, they get corrupt, lose their objectivity and fail to achieve the aims at which they set out to in the first place.