Pros
ADP is a stable company that if you work hard and get the job done, you never have to worry about having your job the next day, next month, or even next year. They take care of the people who take care of the company. The expectations that management have of you aren't unreasonable and are achievable IF you're willing to work. On-site, FREE medical benefits are an awesome addition to relatively competitive salaries, and employees are even still getting bonuses, even with the current state of the economy. Management styles within the company vary, but the management in my group is very good and interested in your personal development.
Cons
A downside of working in a large company such as ADP is that you have limited if not non-existent opportunities for advancement to senior management levels. The raises are small compared to smaller companies, but at least you have a job. And the buildings are over-crowded.