Is it worth joining the union?
YES, of course!
Just compare conditions in unionized companies with non-unionized companies. There is a reason some companies fight unions to the bone - they don't do that for you as an employee.
In jobs where hundreds or thousands of employees do the exactly same job, collective bargaining is the only way to get a fair share of the profit your company is making. Since in such jobs you can't bring yourself ahead with individual effort or qualification, individual contract negotiations would always put you on the short end - since you are completely replacable.