Pros
I thoroughly enjoyed working as a seasonal driver's helper. UPS provides the uniform (except for the cap). It's great exercise, and while you have to be in reasonable shape to do it, it isn't as if you have to be Mr. or Ms. Universe. And the pay in my area was pretty decent ($10.50 / hr, averaging around 7 hours per day). Except for one day, I worked with the same driver for the duration. If you are fortunate enough to work with a nice driver, they really appreciate the help!
Cons
At first, I didn't always know if, or where, I would be working. After a while, though, I stayed with the same driver and our meeting time and place became set. Also, be mindful of the weather. The helpers are in and out of the truck constantly, and the trucks are not necessarily toasty warm! :) If you work where temperatures can dip, I highly recommend wearing thermal underwear beneath the UPS brown.