Pros
Opportunity to work with intelligent, fun people on a fresh and interesting issue. Some benefits, like the metro card and weekly lunches, are nice.
A great place to be entry-level-- as the place is essentially run by 20-somethings and the staff is stretched thin, you'll immediately get substantial experience to put on your resume for your next job.
Cons
The executive level mainly focuses their time on bringing money in, so they're not very connected to the day-to-day operations of the organization. Thus, unreasonable expectations are often placed on the staff, and dissent is discouraged.
The ED is weirdly adversarial toward groups/political scientists who disagree with the organization's proposals. He's been reluctant to assign quantitative projects to female research staffers, which is hilariously ironic giving the organization's main program built around empowering woman candidates.
FairVote has hemorrhaged staff lately, placing a heavy burden on the remaining team. Attempts to hire replacements have been slow.