Pros
Company will always be here. They run it very well (aside from a hiccup here and there...COUGHTNTCOUGH) Numerous employee discounts and benefits. Full benefits for part-timers. Once you make full time in mgmt the sky is the limit, and the pay is great. Part timers face fierce competition and politics. Fortune 50 company (the top 50 of fortune 500). Any tier 1 company will have a discount for UPS employees. All the big wireless companies do for example. You will never be wondering what your boss thinks of you. They will always be forthcoming about it.
Cons
Very physical and demanding job. If you aren't up to it you will be harassed to perform better until you do...or until you quit. Some workers learn they have a "can't get fired" card from the union and slack off, which makes everyone else's job harder. A pat on the back happens far less often than a slap in the face (not literally) Operations people put up the best numbers ever and the performance based bonus checks (MGMT) and Christmas turkeys (Everyone) -- poof-- disappeared, and haven't been back in 4 years.