Pros
Lots of friendly people who you'll build lasting relationships with. Good place to start, especially if you have no experience or are trying to discover what you want to do, since you'll be doing a little bit of everything. If you're on a good client, you'll be working with dedicated volunteers, a decent budget, and have the opportunity to touch many programs. Work-life balance is ok, depending on your manager/team. 401k match is excellent. Office is in a good location. Most employees are dedicated to giving superb customer/client service. Employees rarely get fired (depending on team/unit), you can get away with a lot of slack.
Cons
Units are extremely siloed, to a fault; business units and service units sometimes clash on priorities, and each unit will compete for personal gain regardless of client needs. Managers receive no training, so having an effective manager willing to bat for you is a gamble. Moving up in the company is based on a rigid set of requirements and seniority, and the process is tedious, all for minimally higher pay. Not much growth potential for lower level employees, not unless you want to stay at least a decade. Depending on which team you're on or which manager you have, your work may be unappreciated and you'll likely have no work-life balance as you continue moving up the ladder. Event Services basically starts with zero work-life balance.