What are the benefits/detriments to graduate students joining unions?

The graduate students at my university (a relatively large state school in the US) are considering joining a union. To help decide whether to sign a union card in support of joining the union, I'm interested in more information about how unions have helped graduate students at other universities. What are some improvements to graduate life gained by graduate students at other universities upon joining a union?

I'm also interested in information about how unions have hurt graduate students at other universities. What are some detriments to graduate life caused by graduate students joining a union?

I'm primarily interested in student-body-wide benefits/detriments, instead of student-specific or faculty/administrator-specific claims (e.g. "my advisor treated me better with the union behind me" or "my students have stopped working since they joined a union" is not what I'm looking for).

Answer

I went to graduate school in mathematics at a university with a TA union. I think the main effect was to level off graduate student compensation across disciplines -- so, the union was apparently a great boon to those in the humanities, but I heard that compensation for math TA's would likely be higher if not for the union.

Another benefit to the union was that it instilled (for many) a sense of camaraderie and common cause. Union events were fun, and they served beer. It was a good way to get to know your fellow graduate students.

The main disadvantage was that dues had to be paid, around $200 a year (most of which went straight to the AFT). This is not a lot, but on our salary it did mean something. Dues were mandatory, even if you opted out of the union -- although state politicians have since seen to it that this is no longer true.

Another potential disadvantage is that union dues went (in part) to political contributions to union-friendly politicians. I didn't mind, but this tended to alienate graduate students who were more politically conservative than me.

There was occasional heated rhetoric when I was there, and even more after I left, but overall the union didn't seem to do much good or harm. Mostly, I remember the beer.