I've used agave prior to fermenting on a couple batches. I didnt like it very much. It seems to be less fermentable than the apple sugar and the end result tasted like alcoholic and thin tasting agave juice. Not really bad, but not what I was looking for.
After the ferment, agave can be useful as a backsweetener. It doesnt have as much body as cane and corn sugar, just a sweet burst that dies quickly. So if you have a cider that is a little sour up front with a decent fruit finish, the agave can help the initial attack of the taste without stomping on the finish like sugar or juice sometimes does.