However, like other human institutions, unions can be full of faults as well. Like the businesses they often clash with, unions frequently focus on the short-term and are willing to sacrifice the needs of some of their members to benefit others. They can be corrupted in their single-minded pursuit of their goals.
I have personally seen both of these sides of unions. When at their best, unions can bring people together to solve the problems that would be insurmountable if fought alone. At their worst, they become full of bickering and discontent that focus more on broader politics than the needs of their members.
In the end, I believe they do more good than harm.