Pros- An entire system in place built to protect employees and help them get reasonable pay, and benefits.Cons- Money Vacuum. Bureaucracy. Red Tape. Are you really get what you're paying for with membership? How effective is your union?
Personally, I had a bad experience with one. I tried to report my boss who was bullying me into coming into work when I was sick, and I later found out was slandering my name to other potential employers when I tried to get out and they'd call for a reference. The union said their hands were tied and there was nothing they could do.