Pros
The absolute leader in payroll, and moving to #1 across all of HR. A true desire by most everyone in leadership to look at the long term client relationship and feedback. Great opportunities for personal and professional growth, and fast advancement for those who perform. Willing to make drastic change when the current way isn't getting the desired results. Huge investment made in employee development and training.
Cons
Big company means the bureaucracy and security policies do sometimes get in the way. You will not get rich with stock options working for ADP (unless maybe you're at a VP level or above). ADP is slow to fire bad employees which means some problem people get moved rather than replaced. New product development is slower than it should be. IT & Hosting group can be a pain for internal issues.