I lived on Fort Belvoir for three years from 2001-2004 (as they were putting up all the new housing) and I just visited the post last week.
Fort Belvoir has the newest housing in the Army and actually has a strip mall within walking distance of the new housing on main post which includes a Starbucks, hair salon, dry cleaners and much more. It's like a little town on post. My wife took pictures of it and I'll post some when I get home this evening.
The PX is very nice and the commissary does the most gross business of all the military commissaries (mostly because of all the retirees that live there).
You're only 20 miles from DC and there is SO much to do with family around the area. Even if you're single, Alexandria is a nice hangout.
As for the Route 1 area north of Fort Belvoir, not the best, but BRAC should clean that area up pretty soon.
Through BRAC, the National Museum of the US Army will be housed on Fort Belvoir as well as many of the Alexandria job at Crystal City.
Fort Belvoir is home to two Army command headquarters, as well as 10 Army commands, 19 different agencies of the Department of Army, eight elements of the U.S. Army Reserve and the Army National Guard, and 26 DoD agencies. Also located on Fort Belvoir are a U.S. Navy construction battalion, a Marine Corps detachment, a U.S. Air Force activity, and an agency from the Department of the Treasury. Over 2,000 military families live on Fort Belvoir. Fort Belvoir also supports a larger population living within a 15-mile radius of the post--5,093 active duty military personnel, 20,000 retired military personnel, 1,441 spouses of active duty, and 6,529 civilian employees.