Pros
The pay is competitive and the benefits are exceptional. The work can be very satisfying. Even at low organizational levels you're given a lot of autonomy and not micro-managed. The work schedules are awesome, with many offices allowing you to work a 9/80 schedule to have a day off every pay period. There is plenty of work available, so the job seems very secure. Every weekend off + 10 federal holidays.
Cons
There is always more work than you can do, so you never feel like you're finished for the day. The job can be jading, since you see a lot of the same discouraging situations over and over. Many of the agency employees are very cynical, and it can be hard to push against that trend. Employees in the agency are empowered and generally well-protected legally. As a result, it can be frustrating to watch co-workers do the bare-minimum with seemingly no consequence.