Its up to you if you join a union. Your best off looking into them when you start work as they can be good and they can be a pain as you need the right one for what you do.

Sometimes they can be of help or a hinderence, a lot of people join a specific union for the wrong reason, possibly like in the Royal mail "because everyone else is in it" there then tied to it.

I was in one but it didnt support me at all as I wasn't in their "core" sector.

I should say that they are pretty good if the company you work with values the employees as they can stick up for you regarding your rights and treatement.