ADP is often though of as an old school company who can be very stuck in its ways. They have been working very hard on changing to meet the needs of the Mobile and Millennial populations, while still maintaining their strict data privacy mandates.
They have reduced their Benefits packages to save money, but offer the HSA that is a very useful and valuable benefit.
Depending on where you work in the US, your culture can vary from formal to ultra-casual.
Lots of great people work there, and work very hard. Good friendships can be made.
Lots of positions for advancement of career - you can be there forever if you work hard.
Very large company that can feel disconnected between what Corporate says is happening, and what's actually happening in local regions.