The company is extremely flexible. The opportunity for challenge and growth is limitless. The level of professionalism is second to none.
Cons
The level of perfection and expectation is set on MAXIMUM. (The flip side is they will do ALL they can to help you reach the goals set for the company, as well as yourself.)
The atmosphere is great. Everyone is very friendly. You won't be dealing with incompetent IT staff. Each person is generally expected to admin their own workstation if they're in the technical part of the company, and IT will generally leave you alone but are helpful when something goes bad in hardware or a problem you can't solve comes up.
Cons
The DC metro system doesn't reach all the way to Reston, so the commute can be tough if you want to live by the city.