Pros
Really great co-workers: they do a good job of mostly hiring kind and interesting people. It's also very social; there's lots of activities. Obviously, it's nice to work for clients with broader goals than selling products; it gives the company a warmer atmosphere than your average agency. The scale of most projects is also small enough that you get a good variety and mix in your workload. They also do a good job of refining the process.. there's usually something learned from each project which makes things smoother for the next one.
Cons
The pay is not great, which would be fine if it were due to the non-profit clients, but it may have more to do with the "top heavy" structure of the company. There's a lot of VPs and managers. Also, It's not really a "creative" agency, but more of a tactical one. There's a big emphasis on email campaigns and analytics (which makes sense since most projects are about raising money for non-profits). So if you're looking to make beautiful and conceptual work, this may not be the best place for you. Finally, a lot of people applying to work here are looking for a non-profit or activist kind of environment, but the actual culture is a little more inside the beltway and buttoned up.