Overall, Lisehow and Hill have provided you a good list of advantages to the Westmoreland Hills and Overlook communities. I would add that Westmoreland Hills and Overlook offer some of the most traditionally-styled Colonials in the area, set against the beautiful rolling hills and established landscapes of the area. I like the convenient access to the Mass Ave, Western Ave, and MacArthur Ave (via Dalecarlia Parkway) corridors.
Certainly, lists of disadvantages depend upon what other communities you are considering, as each neighborhood has relative advantages and disadvantages. Aside from Lisehowe's comments, Westmoreland Hills and Overlook are not within walking distance of Metrorail, although the communities have very good access to the Mass Ave corridor and MetroBus. The terrain can make driving somewhat challenging in snow, although our area has few snow events each year.