My sister did it for about 6 months for the firefighters in Australia and found it a real pain in the bum. Those that bleat most about equality and workers rights are often the first to forget it when it comes to their own workers. There was an expectation that to "help the cause" she was expected to put up with a lot of stuff that would not be expected by a decent private employer. However, she did get to hang around with lots of firemen, so not too bad in the balance.
I am currently employed by a branch office one of the largest unions, not directly the by union. I should be on the same terms and conditions but I have been fecking unbelievably shafted - bastards! My trouble is due to the fact my line manager, the branch secretary is a person voted in by the members and she has no experience of staff management or any time for staff management. I really thought that being employed by a union even indirectly would be great but in my experience it is not. However, the pay is pretty good and so are the T's and C's if you have a decent boss.