Pros
You have the opportunity to conduct meaningful research and see your work cited by very important stakeholders in politics, research, policy, etc. Colleagues are very nice and professional. The workplace is very welcoming and affirming! Benefits packages are extremely generous. The work-life balance is second-to-none (average working hours per week are probably below 40 for most researchers, with some volatility during peak publishing periods).
Cons
Because of the org's non-profit status, there is not a lot of extra money available. Do not expect many perks. Salaries are altogether lower than analogous positions at for-profit companies. The Pew Research Center functions mostly as a loose confederation of research teams, which leads to inefficiencies. Higher-ups are trying to create a more cohesive structure, but this can be really slow. Growth opportunities are extremely limited and slow-moving. You are effectively required to leave the org and go to grad school before they will consider promoting you above a certain level (a few have done it without this, but it is rare). Your growth path is confined to a rigid structure imposed by HR, and it is difficult to get HR to recognize valuable contributions that fall outside the bounds of the preset framework.