Public unions have not proven themselves in the same way as private unions have. Private unions were (and in some cases still are) a good tool to protect employees against corporate greed run a muck.
But as we've seen with our over spending governments the inclination of the government is to be overly generous with people like teachers and that's now being abused.
This isn't a liberal/conservative issue. FDR is still the most liberal president we've ever had and even he said public unions were "unthinkable and intolerable". The head of the AFL-CIO was against public unions in the 50s.
(Facts taken from the New York Times: http://tinyurl.com/47u53dq)
Here's the exact FDR quote...
"[a] strike of public employees manifests nothing less than an intent on their part to obstruct the operations of government until their demands are satisfied. Such action looking toward the paralysis of government by those who have sworn to support it is unthinkable and intolerable."
Isn't that exactly what's happening right now? Look at this quote from the article...
"This is a quasi-legal power grab,” said Richard Larkin, a teacher at Classical High School. “You want to pick and choose teachers. Well, we will not be bullied."
Isn't picking and choosing teachers exactly what has to be done in order to keep good teachers and get rid of bad ones? How out of control do you have to be when you don't acknowledge the fact that the school should be able to pick which teachers it thinks are best?