I stayed at the Residence Inn Ballston for three nights to visit family. As someone who usually visits Arlington once a year, the Westin was my "go-to" hotel, but the new Residence Inn is a strong competitor.
I booked a one bedroom suite for $125 a night, which was a good value. Especially appreciated was the kitchen area, which included a full-sized refrigerator, plates, and silverware. The suite had a large living area and a decent sized bedroom.
I especially enjoyed the complimentary breakfast off the lobby, which included hot dishes (eggs, make-your-own waffles, french toast), breads, fruit, cereal, juices, and coffee. It was nice that the selection rotated a bit each morning. Another bonus was the complementary internet access. The hotel also offers a light dinner Mondays through Wednesdays (too bad I stayed from Thursday to Sunday), but the menus are posted in each suite.
Unfortunately, there were a few downsides: the hotel only had two elevators, so it frequently took a while for one to arrive. Also, the exercise room and pool were both quite small.
Overall, the Residence Inn is a great choice, especially for families or extended-stay travelers. There is an attached garage, and the Ballston metro station (Orange Line) is just a 10-12 minute walk away.