What are the pros and cons?

I've never personally been helped by the union. Both times I needed them, I was untenured and therefore, the union could only give me moral support. The union is good when it negotiates things in the teachers' favor, but sometimes they're not much help at all.
The worst part of the union is the huge dues we have to pay. I do feel though, if we weren't unionized, the administration would totally take advantage of us.

I'm in a union now and i'm sorry its better. Its not heaven on earth by any means, but I can't just be sent out the door on a stupid WHIM without DUE PROCESS and that's all anyone deserves. In my experience, unions are only designed to make sure you get what is offered, not much more and not much less. So if they offer a dollar and a sandwich and sign on the dotted line, then the union is there ot make sure that happens. If it does not then the union is there to help you file a grievance as to why not.