To get to Dulles Airport from downtown Washington, the Metro is an excellent choice. Most of the ride is above ground, which makes it an interesting ride to look out the window.
We loved seeing some interesting architecture, we saw the police checking people, and we even got to see a plane landing as we got closer to the airport. It's also a very affordable way to get to Dulles, and with all the interesting sights along the way, the ride doesn't even feel that long.
Have you taken the Metro to the Dulles Airport?