The hotel itself is very nice. We recently stayed here for a visit to the museums on the mall and got a huge 2BR with a kitchenette for a very attractive price - with a free breakfast included. The pool/hot tub was great for us and our kids. It is a very short walk to Air and Space and the American Indian Museum (great for lunch BTW) and a not too long walk to everything else on the mall. The staff was polite and helpful. But for a few caveats it would have been five stars...
1) First and foremost be aware that the area around the hotel shuts down at night and on weekends. It didn't in any way feel unsafe - just be aware that unless you are willing to eat pizza for dinner you'll be taking the metro or a cab to go to dinner.
2) The beds were not up to usual Marriott (or even Courtyard by Marriott) standards. Not a problem for the kids...but my back was feeling it after two nights.
3) The breakfast definitely skews southern - there was nothing really that wasn't sweet or fatty (e.g. nuts covered in syrup, no plain yogurt, no lean meats, etc.).More