Durarara is based on a long running light novel, like baccano and it's authored by the same author. And like baccano, it only covers a small part of the whole series.

Durarara got 26 episodes, the latter half will have an overreaching plot that will reach a conclusion, but it won't actually reach a real 'series end' at the end of the run because there are still other novels.

So yeah. I can only bite my nails at the end of the series, because it's ending the series when for me, it's getting to the really awesome part.