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