Ups and Downs just like any other job
Pros
For the most part once you get in as a fulltime regular carrier pay is pretty good. I have been a regular for 6 years now my base "salary" if you will is about 52,000 which if you know anything about the post office you know... what you make at the end of the year will be more due to overtime both voluntary and forced. Moving up is not terribly hard as long at you are willing to be a 204b (supervisor). The longer your in the easier it gets due to seniority and what route you get. Great benefits from health and pension, thrift savings which they match 5%, vacation time which at top level is a little over 5 weeks a year and you can carry some over to the next year, sick time which you earn 4 hours every pay period and they don't expire through out your entire carrier ( i'm up to about 2 months of sick time).
Cons
The beginning of your career will be terrible! There is a huge turn over rate due to the fact when you first start your a CCA. Which other terms means less pay, less benefits, and terrible work to home life ratio. And it can take YEARS until converted to regular or it can take 5 months in, there is no saying. Your job highly depends on what kind of management you have in the office. Hard A** with something to prove can make your time here miserable. As far as advancement good luck getting a job you really want in here unless you become management first and kiss some butt or know somebody. Which sucks because their are a lot of unique and cool jobs that you would never think of the post office for having.