Pros
UPS has a very strong union and as long as you don't move in to management, you will work under their protection. This can be extremely helpful in cases where they somehow skip your paycheck (which they will).
Cons
You will not be rewarded for hard work. The harder you work, the harder they will push you. You are not allowed any days off (vacation or sick) for your first year of employment. When I was hired I made it very clear I would be gone for a week and was told that would be okay. When I returned and didn't receive my paycheck, I found out I was terminated without anyone telling me or my supervisors, so I technically worked a week without even being in their system. My supervisors got me back in the system but I didn't receive the paycheck for that week of work for another three weeks. Pay rates are flat throughout the building so if you are a loader, you'll get paid the same amount as people doing less than half of the amount of work. Supervisors are power hungry and will not treat you as human beings. In the few months I worked at UPS, I saw a total of 4 paychecks go unpaid. Moving into other jobs within the building is completely seniority based. It doesn't matter how good a worker you are, all they care about is how long you've been there.