The Udvar-Hazy Center was opened because the Smithsonian needed a place to display more aircraft, and boy, did they have more aircraft to display! Old planes, newer planes, war planes, commercial planes, spacecraft, even science-fiction movie models. If you like planes, you should definitely go, and even if you don't, you'll probably find something to enjoy. Myself, I was unexpectedly moved by standing next to the Discovery shuttle. a behemoth that made it to space and back, safely, so many times. I also enjoyed looking at the restoration hangar, even though not much was going on that day.
This museum is very accessible. You can get upstairs, downstairs, pretty much anywhere you want to go, although some of the ramps are of necessity very long. (There is also an elevator.) My most major complaint is the restroom. We prefer to use a family restroom for our daughter, and theirs is very small and usually has a long line, being the only one in the building. It also looked like the gift shop might be challenging to navigate, although we didn't go in.