Pros
Happily worked at PSE over 5 years, and plan to be there many more! Small company (with all the good that implies). Opportunities to work on many diverse, interesting, and challenging problems. Many different opportunities to contribute, and/or to lead tasks and projects. Co-workers come from many backgrounds to form interdisciplinary teams - which provides a lot of different perspectives, and opportunities to learn. Lots of camaraderie among teammates makes for a fun work environment. Many employees socialize after hours, which adds to the fun. Occasional opportunities for travel (some international). Very competitive pay, and superior benefits compared to similar companies. Lots of time off, great healthcare, 401K, ESOP, flexible work hours, active professional development programs, fantastic support staff and management.
Cons
A few bad apples and complainers sometimes distract from an otherwise great place to work. Much of the work is for the government, so there are occasional contracting and funding delays, and super-busy periods, which can be stressful for some. Small company (with the few drawbacks that implies).