Going West on the Custis along 66 is great if hill climbing is your thing. Once Custis ends at Four Mile Run/W&OD (can't remember which it is there, but it's all the same trail) you have a gentle downhill coast all the way to Shirlington. Here, you have to cut through Shirlington, stop by Cap City Brewpub to refresh, then climb up and over the 395 bike trail overpass and down into Parkfairfax. A 1/4mile along a street reconnects you to Four Mile Run trail on the other side. That last part of the trail is grungy - along the creek which is paved and underneath a series of bridges - Route 1, railroad, GW Parkway as you suddenly pop up on the Mount Vernon Trail.
I don't know if I would do it for the scenery overall. But the Custis trail's hills are a worthy adversary. You can do it in reverse and the hills are much less difficult.
Just found this thread, so that's why the old reply. Custis is a beast going west from Rosslyn - Glebe or so. It's a little hilly after that, but once you go under Glebe you only have one or two bad ones left. Going east is a lot easier.
Matter of fact, Custis' hills are why I haven't committed to riding into work yet - I hate the idea of having to conquer those hills every night on the way home. That, and I can't figure out which bridge to take from Rosslyn - I'd like to take the TR, but then I don't know where to go once I get to the Kennedy Center. Could take Key, but then I'm doing M St. the whole way.
MDS - you commute on M through Georgetown? Just seems like a good way to have a close encounter with some needledick's 5-series.