Ventajas
The management staff that my team worked for was full of praise and support, was clear about their expectations, and gave us much creative freedom in our work. I wish I could work there again but I am not in Atlanta anymore. It's easy to meet people across the company. They are very friendly and some will go out of there way to help you in and outside of work. The Atlanta headquarters building (construction in ~ 1994) is a superb work location surrounded by the sites original woody trees, beautiful architecture inside and out, lots of windows looking out on the forest, on-site walking paths, outdoor court yard, affordable employee gym, very good cafeteria, and many of the latest green building perks ahead of its time.
Desventajas
UPS is very much a chain of command type company. Just make sure to keep your supervisor in the know about who you are calling and what you are up to. Don't do anything behind any body's back.