Vantaggi
EPA is statistically one of the most effective agencies in the federal government. Every tax dollar spent produces exponentially more savings for taxpayers in the form of improved health outcomes. You will feel you are truly helping people because the results are tangible. My own personal experience was also good because my supervisor and all of my co-workers were phenomenally talented and had deep technical expertise. My supervisor was also very supportive. Overall, the agency culture was solid, as one would expect from a culture dominated by scientists and engineers as opposed to soft-skilled political lackies in agencies that have less tangible missions.
Svantaggi
Your enjoyment of the job will depend greatly on the content. I worked in permitting, which was mind-blowingly dull. I can't imagine anything more boring. Every day was exactly like the day before, with no end in sight. Think Groundhog Day, except you never move from your computer or leave your room, and the only thing that changes is your age. A couple of my co-workers routinely fell asleep at their desks every single day. I lasted a couple of years but chose to resign before my eyes crossed permanently. That's not a ding on the necessity of the job, it's just the reality of doing it. However, the work in other sections looked to be very interesting. I probably would have stayed if I worked in a different technical area.