I was just in one of these townhouses last week, visiting someone.
I can tell you, it was COLD in there! But I don't know if that's because it was poorly built or because the person I was visiting just keeps her heat really low. They're certainly not new townhomes, though, so whatever you do make sure it's been well-maintained, otherwise your costs will go up.
They are pretty convenient to Metro, and to drive just about anywhere. And I definitely agree that the proximity to Metro will keep the value higher than in some other places. But I'm not really crazy about the area in general (I live about a mile away, off Nutley), personally. Fairfax City is great on the whole, but Fairfax Circle is kind of dumpy. And Fairfax Circle is kind of in your front yard if you're in those townhouses. In addition, there are no sidewalks on that side of Rt. 50, so it's not a very walkable area. You're fine walking within the immediate neighborhood, but it's not all that huge and outside of it, you're out of luck.
It's not an unsafe area. Not the absolute best in FFX County, but not sending you running to lock your doors and hide, either.
