I have stayed at the CP Dulles a couple of times heading out on international trips. The hotel is a short shuttle ride from the airport. The hotel itself is nice enough but a bit worn in the rooms. Staff for the most part are friendly.
Hoolihans is a great place to hang out, eat, have a drink. There are several places around the hotel to walk to, including a Giant grocery store on the north side of Kohls. Place has everything you need.
My biggest complaint is the "fitness center" which is more like a closet with a bit of equiptment in it. Didn't even attempt to use it.
Overall I would probably try one of the other HI properties around Dulles next time I come through.