While on a work assignment, I lived in the Candlewood Suites in Herndon for nearly a full year.
The staff here is the friendliest and most accommodating you will encounter. The hotel has a gym, free wifi, a computer room, BBQ grills, and a free laundry facility! The rooms are also quite comfortable and newly renovated with brand new counters, electric stoves, refrigerators, and cable service. For the very fair rate, you won't find a better deal anywhere around. I highly recommend if you are visiting the area on a budget for either a short or long stay.
For me it was also an ideal location due to the close proximity to; Dulles airport, Reston (where I worked), and nearby shopping centers with plenty selection of food and drink. If I'm ever back in town, I will most definitely return here.