Pros
All of the individual UPS Stores are owned and operated by franchises so your experience may vary. My experience was generally favorable as the owner of my store was competent and fair. It was better experience than fast food or retail and therefore probably more valuable on a resume, in that the basic business skills you (can) pickup can serve more than flipping patties, chasing grocery carts or stocking shelves would. A good portion of the customers are business owners themselves which encouraged a more professional environment. I would recommend it as a good early/first job. A number of fellow employees were mothers of older children who were starting to wade into the workplace without jumping right in and it seemed to work for them. Obviously all managers are different, so again, YMMV, but they were pretty reasonable about time-off and scheduling.
Cons
There isn't much room for promotion beyond manager of the store. Most UPS store owners in my experience are “frenemies” so it's not you could advance beyond the number of other stores your store owner happens to own. Also, as with many customer-facing jobs, the customers come in waves. So the job is typically long periods of sheer boredom trying to come up with activities to be productive punctuated by periods of terror trying to service a tsunami of impatient customers that come in all at once.