Francis Poretto has written one of the best essays about the current (and hopefully future) state of trade unions I have ever read.
I am of the opinion that Unions as an institution are simply a manner of free association; and in that association voluntary collectivism can be used to attain goals. The problem lies in the special protected and privileged status of various unions. Why does it exist, and what should be done about it?
Personally I believe that all legal privelige, and co-ercive power over membership should be removed from all laws relating to unions and trade practices. At that point the unions would be exactly what they should be, voluntary trade associations run for the benefit of their members.