Pay is very low for the job market. Paychex apparently bases it's wage on the banking industry, when payroll specialists/customer service reps are essentially HR and Accounting consultants. The work doesn't match up to the work load - high turnover is the result.
Semi-Annual Bonus is lame - not based on personal performance, but office performance. High turnover and undervalued employees results in poor customer feedback and inability to reach arbitrary corporate goals.
Annual reviews / merit increases are minimal - typically 2 - 3 percent.
Inadequately managed. Supervisors are expected to be full-on business managers, and actual managers want to delegate and "teach" the supervisors.
Interviewing is arduous (often 3 weeks+ with 5+ meetings) and mgmt still hire poorly qualified or poorly motivated candidates.
Likely typical large corporate complaints. If you're a "glass half full" person Paychex is probably a good fit. Otherwise, think twice before making the plunge.