Pros
If you are in the union, it's a cushy job that you get overpaid to do. I suppose that would be a benefit to some. The benefits are really good (and totally free!) and you get a lot of vacation time. Unfortunately, I think those will be fleeting giving the current economy, 12k layoffs in the pipe and a union contract that expires in 4/2009. The opportunity for overtime has led to some obcene paychecks.
Cons
For me, being in the union is a negative. The union holds people back that want to do a great job. It takes away all incentive to do so. It also keeps people in a job that really don't have the skills/abilities to excel at it. Also, the union takes around $625 in due each year. Depending on what department you end up in promotions are either easy to get (assuming you are working towards that goal) or impossible (as has been my experience). I've been trying to be promoted for nearly 10 years now. I've seen many friends and colleages promoted. Taking a promotion to the management side of the fence nets you a 10% pay hike over your previous title. Managers also get a bonus based on performance that is 10% of base salary or better (based on company-wide performance). A downside is managers pay for their benefits (just like most other people in the real world).