I'm against some unions. The UAW for example is a cancer and was a significant part of the reason why GM and Chrysler went bankrupt. Others I'm more torn about. Teacher's unions do some good things (such as mandated minimum student to teacher ratios) and some bad (hiring and firing based strictly on seniority for example).
I do not believe anyone should ever have to join a union or pay union dues to get a job, and I support anonymous votes on union issues so workers can't be intimidated into voting a certain way.