Ventajas
R/GA is a place that can really pump up your resume and experience. Work with top brands, smart people, get to do work that's award winning and generally "best in class." Pay (especially strategic / account / tech departments) is better or comparable to other agencies and tech companies. If not the name, the experience at R/GA will open a lot of doors later. If you learn how to navigate the politics / people, you can basically mold your own career in terms of things you work on and how fast you progress. The culture can be really social, if that's what you want it to be.
Desventajas
Everything good that's been said about working at R/GA comes with a caveat. Nothing good at R/GA happens unless you learn how to navigate the politics, drop the team-centric spirit, and spend a lot time pushing your own brand and agenda. If you don't, it's easy to get lost in the shuffle and feel like you work on bland work with no visibility. Unfortunately, this path to success leads to an "everyone for him/herself" culture, lack of respect between employees and leadership, and roadblocks/frustration because people can't or won't work together as a team.