Just stayed at the Marriott Suites right outside Dulles for business and I have to say it was an excellent property. Start with the rooms. The suite concept is nice, as it gives the illusion of more space. My room had a bedroom, living room, and bath in an all-connected, triangular configuration. The bed was comfortable and the air conditioning kept the room (intentionally) frigid at night.
The work area was one of the most comfortable I'd had in a long time and the wireless offered adequate performance. At times, it seemed pokey, but nothing too serious to complain about.
The hotel does not have a Concierge Lounge, but does offer a complementary breakfast buffet in their ground-floor restaurant to Elite members. In many ways, this is better than the typical Concierge Lounge experience, since you get a better meal. What you lose, of course, is the all-day snack/drink service that the Lounge typically provides.
But what I really, really liked best about this hotel was the location -- not just its proximity to Dulles and all the businesses that cluster around the airport like teenage girls at a boy band concert. The Marriott Suites sits atop a mini-strip mall that offers no fewer than a dozen restaurants. In addition to the standard TGI Fridays, there's also a Subway, a Starbucks, Indian, Chinese, Japanese, Thai, Korean, Mediterranean, Pizza, Ice Cream, and a Sports Bar. The ones I tried were pretty good and most were accessible with a 30-second walk from the hotel.
When I'm back in the area, this is definitely where I'll be staying.