I can't speak from much personal experience, but I have several friends who live in Ashburn (immediately west of Dulles), and have been out to visit them several times.
- Traffic can be AWFUL. And, as others have said, if the weather gets bad (esp. if it snows), traffic just becomes exponentially worse.
- There's a ton of new construction (i.e., sprawl) moving west along the toll road. The arterial roads have a very hard time handling all the increased traffic (esp. west of Dulles, much of that area was fundamentally rural just a few years ago).
- If you're going to be working near Dulles, get a place to live fairly close to there, unless you really like sitting in the car. And, under no circumstances, agree to live in Maryland or D.C. (i.e., on the other side of the Potomac). There are very few bridges across the Potomac, and those bridges become tremendous traffic bottlenecks. One couple that I'm friends with moved out there, and built a house in Gaithersburg MD, because they were both working in nearby Bethesda MD. Within two years, they'd both changed jobs, and were both working in Tysons Corner. Their commutes became absolutely unbearable (at least once a week, an accident or bad weather would make the commute 2-3 hours long), and they eventually threw in the towel, sold the house in Gaithersburg, and moved to Ashburn.
- Housing prices are just nutty. It continues to be one of the hottest housing markets in the U.S., and buying even a small townhouse is amazingly expensive.