There is also the Washington Flyer shuttle bus from IAD to West Falls Church Metro which can be a good option for getting in and out of the city. They're working on building a metro line all the way out to IAD but I'd guess that's about a bazillion years from actually happening. IAD isn't quite as far out as a lot of people think it is, but if I had to make that drive back and forth to the city during rush hour very many times I'd get a little crazied up.

DCA is far more pleasing to the eye than IAD. Relatively new terminal that's almost as long as the runway. Security is usually not too terrible, but can have it's moments like anywhere else. And you can walk to metro.

IAD on the other hand... while the main terminal is I guess some sort of architectural marvel, the mid-field terminal where people actually fly in and out of on UA was built as a temporary terminal. That was many many years ago, and it's still there. Low ceilings, poor dining options, etc. But the bar in the RCC makes everything tolerable.