Pros
Very flexible: Could often do my projects from home, design my own research, make my own hours. As an intern, great work/life balance- you won't be working insane hours here and if you stay too long, you will likely get people telling you to go home. Incredible mentorship: Research fellows are all there as resources and want to help interns succeed. Workplace environment: Beautiful buildings, wonderful coworkers and co-interns, hustle and bustle of international students arriving for their classes. Also: Various workshops on research techniques, brown-bag lunch sessions on international events, free access to publications on top international security events, and an amazing library.
Cons
If you are an intern, you are not getting paid and that is TOUGH when living in Washington, D.C. Some research professors will have more concrete tasks than others, but you have to be really flexible in terms of what projects you are put on. You may know a lot about them or you may know nothing. This was great for me, but may be tough for someone looking for something more specific and concrete. Very conservative environment, which may work for some people and not for others. Not situated in the best area of D.C., so be careful if you work late.