I second most of the comments about the IAD Marriott. The building has a dated suburban design that's very spread out. Some may not like the distance required to walk from the front desk to rooms in the back wing (or from there to the CL). Though ample parking and lots of side entrances mean that once you've located your room you can park close to it.

The rooms were updated several years ago but the decor is very bland. I've gotten rooms of varying size, ranging from slightly cozy to reasonably spacious.

Service has always been good. Not exemplary, but solidly good.

Per TrojanHorse's post above, yes, the IAD Marriott is super close to the airport terminals but pretty much nothing for eating, drinking, or shopping other than what's inside the hotel itself. You will have to drive for anything else. Whereas other hotels in nearby Herndon and Reston offer food and shopping within walking distance.

Despite that I have been satisfied with the IAD Marriott even for multi-night stays. When I'm already organizing my stay around renting a car because I've got to go here, there, and there within the local area, it's no problem using that car to go out for dinner or bring food back to the hotel. Plus, parking is free. Without a car I would pick some other hotel for a stay longer than 1 night.