Pros: You will get paid more and have a lot of benefits.
Cons: (1) You have to pay dues. (2) You do what the union says whether you want to strike or not. Some of that is changing, though. (3) The unions demand so much out of the companies in terms of giving high pay and benefits, that it's not good for the business and they can end up going out of business or doing serious lay-offs. (Enter GM, okay?) (4) The union is your boss. You don't get a say. Frankly, I'd rather have a few bucks less per hour than get laid off or have the company go under.
(5) The union is just one big politically driven organization designed to control companies, its workers and make big bucks out of it.
Done. I would never, ever work for someone who had a union.