I'm interpreting your question as to the benefits and drawbacks of working in a unionized place, not the act of joining or not joining, if you work in a place that already has a union. Legally, at least in the US, you never have to "join" the union, but you can be required to pay dues.
I think the main benefit to having a union is that you have a contract that contains negotiated terms and conditions of employment, that cannot be unilaterally changed by management. This brings with it protections against (most) arbitrary decisions and actions by management. Also, there is strength in numbers, and you have a greater voice when you speak collectively.
Second, assuming there is a 'just cause' provision in the contract, you have protections against unjust discipline or discharge.
Third, in the event of a dispute, most union contracts have a provision for a grievance procedure, with binding arbitration as the final step. In a non-union world, if there is a problem to be addressed, it's usually you (the worker) and your boss, and maybe the next higher-level boss, and someone from Human Resources. Sometimes the H/R person is really on the ball, and knows how to balance the interests of the boss and the employee, and act as an advocate for the employee to the extent that is necessary. Sadly, that is the exception. Most HR people, whether in a union or non-union shop, see their role as supporting line (operating) management, and that sometimes (often?) means giving the employee the short end of the stick. I've seen it happen over and over. So having a union rep. with you in these kinds of "meetings to discuss your problem" can be exceptionably valuable. Why? Simply because the union rep does not work for the Company, so he has little or no vested interest in 'getting along' with management. He is free to advocate your interests as strongly as he thinks is prudent. Knowing this, the Company is far less likely to try to railroad you, in the event of any dispute.
Some of the previous answers have touched on the idea that a union contract protects slack employees as well as average and good ones. I have 2 comments about that.
First, that deal has limited value. A union probably can protect a poor performer for a little while, but if poor performance continues, that person eventually will be gone, union or not. The term "just cause" does NOT mean you cannot be fired, and trust me, many workers are fired for a sufficient reason.
Second, I have seen marginal workers in a non-union work place keep their jobs because of non-merit reasons. Maybe they befriend the boss (like by helping the boss's kid in school) or maybe the boss is the type who likes to be perceived as a nice person, and is therefore loathe to discipline or fire anyone.
Downside to a union? Well, obviously you have to pay dues, and sometimes an initiation fee, and sometimes assessments along the way. Most members do not pay enough attention to where their money goes, and this can help create an environment of corruption within the union leadership, which of course is very bad, as it gives all unions a bad name. Second, while some unions permit individual negotiation of pay above the union minimum, such as in sports and movie acting, under most union contracts, you cannot negotiate directly with management for higher pay for yourself. Some people would see this as a negative, but I'm not sure the average person would.
Another downside is that unions are run on a majority rule concept. Sometimes the majority simply isn't right.
I hope this helps, and I apologize for the answer being way too long.
Answered By: Carlos R - 9/10/2006