In common with all of the other Smithsonian properties, this museum is free. It used to be even fuller but the expansion to the Udvar Hazy Centre (out near Dulles airport and also well worth a visit) means that some of the aircraft (e.g. the Enola Gay) are now housed out there. Even if you are not a museum person, you will find time passing without you realising as you meander around this excellently constructed record of human flight. I've done all of the Smithsonian museums in DC and this is easily the best (though the art galleries, the Natural History Museum and the Museum of American History are also fantastic).
Don't just leave it until a rainy day in DC - put it on your must sees. (Tip - if you are on the Mall seeing the other sites, come here for a couple of hours in the heat of the day to cool down. All of the monuments on the mall photograph better in the morning and afternoon anyway - the light around midday in DC is just too harsh.)
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.