Pros
ADP is a pretty solid place to work long term. There is low risk of getting laid off, however an even lower risk of you being promoted to anything but what you were hired too do. IF you singled out for promotion, it seems like your constantly the target to get those, to fast track you into the managment "team", regardless if you do well.
Cons
There is no career path outside of managment. Sure there is always the Senior "whatever your title was when you where hired" but that doesn't change anything, and at most you might get a few dollars more on your pay check. Don't expect promotions of REAL merit, and don't expect annual wage increases that are anywhere near cost of living inflation. You WILL be making less in 2-3 years than when you started even tho you got (small) raises each year.