I've seen good unions and I've seen bad unions. From my experience, it all depends on who's running it. Unfortunately, corruption is just to easy to get away with in a union. Same goes with idiocy.

Good union:
When I lived in south Florida, I saw the teacher's union fight for the raises the teachers were supposed to get but didn't, all because the school board built a new headquarters with a nice pond and palm trees lining the drive, that went over budget.

Bad union:
Here in Houston, the firefighter's union has fought to inflate salaries, benefits, and pensions for years. I'm not saying firefighters don't deserve to be compensated well, they absolutely do. However, for years we had senior firefighters driving ambulances and making as much as a captain, with the pension to go with it. Now almost 50% of the fire department's budget goes to paying retired firefighters.
When I work for Chilis doing new store opening and remodels, anytime we did a store in a union state up in the north east, I had to hire 3 union guys to do my job for me, and it took the 3 times as long to do it. How is that beneficial?