Vantaggi
By far the best benefits you can get for only working 17 hours a week. Job advancement all depends on timing (everything is seniority based except management) but is very much worth it if you can become a package/feeder/air/combo driver. Great work out. Teamster (union) influence (in most areas it's pretty good - some centers I've worked at were absolutely terrible as far as the company/union relations)
Svantaggi
Very physical - at my current center (of 5 years) I've seen 4 or 5 military reserve employees quit due to physical stress (they actually walked out mid sort). When working a night sort (local sort), summer temperatures in the trailers average over 140 degrees. During the winter, preload sorts (morning sorts) can be single digit temperatures depending on where you live - and you WILL work in it. Though I listed it as a pro, seniority based job advancement can be terrible in specific cases. You can only transfer as a part-timer by means of education, death in the family, or mortal health/sickness. Transferring is impossible if there aren't any openings.