At the IAML congress in Antwerp 2014 Hartmut Walravens and Carolin Unger from the International ISMN Agency gave a presentation on the history and success of the ISMN. The text gives a good overview of the benefits of the ISMN and how the number is being used. It was first published in Fontes Artis Musicae, Volume 62, Issue 1, page 26-36. You can see the English version here and the Spanish version here.