I like unions but I when people ask about any particular union, I basically dislike them all. I don't like the AMA (doctor's union) becuase I can't see a doctor in a timely manner because of those bastards. I don't like the MBTA's union because the transit system sucks and they pay the bus drivers $25/hour. I don't like either of the two major teacher's unions because, politically, they are major roadblocks to improving education. I don't like firefighter unions because they're overpaid in almost every state.
So how can I like unions?
The thing is that too much of a good thing can be a bad thing. Unions are good because they help workers. Unions are bad because higher wages, shorter work days, better benefits, and lets not forget racketeering, defending racial discrimination, and stuff ballot boxes, have a cost to society. When unions are too weak in an industry, they can help workers a lot (SEIU might qualify here). When unions are too powerful consumers get it stuffed up their asses (students, MBTA riders, etc.) in addition to the usual downsides of concentrated power (corruption, aforementioned labor rackets, etc.).
Naturally the best known unions are the big ones. And the big ones are the bad ones. So it's not surprising that I hate most of the unions you have ever heard of. But that doesn't mean I don't like unions.