Are teachers' unions helpful to US society?

I’m generally of the opinion that unions once served a great cause, but over time have gained sufficient power that they are in need of reform. I think this reform can be very difficult to achieve. I watched “Waiting for Superman” last night, and found myself thinking about teachers’ unions a lot today. This documentary points out significant problems with these unions and how they actually serve to hinder education. One specific example is that tenured teachers that are horrible at their jobs are very difficult to remove. This can have a terrible effect on the students that are subjected to them.

That’s all I’ll say about the negatives. I’m aware of a lot of the benefits of unions in general, and they do provide many worker protections and benefits. I’m not anti-union, but I am pro-union reform.

What are your opinions on the subject?