The CP Dulles is very convenient to IAD and provides a regularly scheduled shuttle, saving you the cost and hassle (more on that later) of taking a taxi such a short distance.
Be sure to ask for a Tower room. These rooms are a bit nicer and much quieter than rooms in the front section of the hotel. The staff is always quite helpful and generally cheerful.
One word of caution: check in lines can be a bit long if there are flight cancellations at IAD as people seek shelter for the evening.
When looking for a place to eat, Houlihan's has a decent menu. However, if you're up for a 10 minute walk, cross the street to the shopping plaza (one with the Marriott Suites) and explore its restaurant selection. I can personally recommend TurCuisine - authentic food, great staff, and generally quick during dinner hours. Friday's has the only late night (1 - 2am depending) bar in the area. If you're jet-lagged and can't sleep it may be your saviour.
As for the taxi hassle: cab drivers detest us travelers who take taxis the 2 miles from IAD to Herndon (location of CP Dulles). It occasionally will spark a confrontation as they are hoping for fares into DC. Don't tolerate this behavior! If they return to their airport within 30 minutes they are allowed to resume their position in line.