Pros
The best reasons to work at Paychex are the opportunity for advancement and the job stability. Paychex has never had lay offs, and unless you are an idiot, you cannot get fired. It's a great company that does very well, you know they won't be closing their doors anytime soon. People sometimes take Paychex for granted, and then things like this happen to the economy to make you appreciate your current employer. I have been there 10 years now, I went from making $24K to $100K+. I know many people that started as a receptionist or a temp and is now a branch manager or zone manager. Great place to start out of college.
Cons
Some of the downsides are the way promotions are handled. It's all politics, as I'm sure every office is. They need to pay their operations people more though.