I have not been into IAD but our flying club has made two trips. From an e-mail regarding one of those trips...
You're right. I got it backward. Signature is closest to 1R/19L and Landmark is closer to 19C/1C. I'll correct my previous post.
I was a passenger on this trip. If you're flying IFR, it's a non-event.
If you're flying VFR, you've got the whole SFRA thing to deal with and I'm sure you're aware of that.
Make sure to have an airport diagram with you. Of course you can't count on getting the runway near your FBO, so you have to be ready. We left about 3-4pm and got a taxi from the NE GA / Customs area of J1-Right J-Right B-Left Z-Left Q, hold short runway 30, all the way across the airport and mixed in with an Airbus in front of us and a 747 behind us for part of the route. The taxi was a little hair raising because there's lots of choices for turns and they're not all marked well.
Listen to departure instructions if you're VFR. If they tell you to take off, turn on course, don't fly the runway heading because you're holding someone else up.
I think that's it's for the flying part
Getting out of Dulles to DC is easiest if you have a rental car. Take the Dulles Access road and there's no toll. Take the wrong road and you could wind up paying $9 for 1/2 mile drive, but at least they take credit cards.
If you want to save taxi fare and you're OK to ride the metro (I am), the new Silver Line ends at Reston Station right now. That's only 7.3 miles from Dulles, so maybe it's only unpleasantly expensive instead of feeling like you've been mugged and there might be a bus or shuttle available from the FBO. Of course, once on the Metro, you have one transfer and get off at Navy Yard about 45 mins - an hour later.
Another alternative would be BWI, which has an Amtrak station at the airport, You'll have to check the cost of the train from BWI to Union Station.