GMB, UNITE, AMICUS...
I was in GMB whilst in my previous job. Had been paying a tenner a month for years, often wondering why I was doing it...now I know why.
Had a problem with personnel and without the union, I'd probably have been screwed over.
The union rep knew the system inside out and as such, got me out of, potentially, a very very sticky situation.
Ok, not all reps will be as good as the chap I had, but in the world of crewing/ops, it's always best to have that back-up. One day, you may need it.