Pros
R/GA truly has a strong opinion on where the marketplace is going and what agencies need to do in order to deliver against the unmet (and often unidentified) needs of its clients. You will have all the tools you need to succeed and the quality of brainpower or technology available to deploy will never be an impediment. The agency sets the bar when it comes to collaboration and I found the environment truly ego-free with the goal of doing the best work a shared value held by all.
Cons
The agency leans a bit too hard on Production but given its heritage this is understandable. There isn't as much structure as you might experience at the big traditional agencies and some people may find that disorienting. You are expected to get smart quickly and be self-sufficient. Only the best will succeed here.