Nah, not worth it.
Some people here are saying it's like an insurance policy in case you need it to support you, but I don't see how the student union can support you any more than just give you advice. They can't circumvent the rules of the university, and they don't have tutors. If you need to form study groups, you won't need a student union membership for that either. Advice isn't worth $200+ a year, you can come on FSN and get it for free.
However, if you believe you have a moderately high risk of becoming depressed or anything that requires counselling, I am pretty sure the student union provides that, and this would be the fiscally conservative way to get it (although I'm not sure about the quality of them, compared to private counsellors).