Udvar-Hazy Air and Space Museum is well worth the trip. My friend and I drove out there yesterday. It had been on my list of places to visit for far too long.
The museum was much larger than I expected. It's about the size of the Air and Space Museum on the National Mall, but much more open.
As with all the Smithsonian museums, admission was free. Parking was $15 though, some of which probably gets kicked back to the museum. This gets no complaints from me; they deserve it. I think the parking fee is also there to discourage people from using it as a parking lot for Dulles Airport, which is literally right next door.
I was very impressed with the way they had all the aircraft lit. Most museums seem to be satisfied with harsh top lighting, but not here. They had the undersides and crevices illuminated so everyone could appreciate (and photograph) the details of things like air intakes and landing gear. If you ever want to learn how to properly light an aircraft, you need to check it out.
All in all, I only wish I'd allocated more time, and that I'd brought a wider lens. I only had my usual 24-105/4 with me, but the 17-40/4 seems like it would have been very useful in some of those spaces. On the flip side, the image stabilization came in very handy in all that low light. I generally had to shoot at ISO 800 and was getting shutter speeds below 1/30. Thanks to the reciprocal rule, this should be fine at 17mm, but would be pushing it at 40mm.
Anyway, whether you bring your camera with you or not, don't put it off like I did. Go check it out!