It is not easy to find insider discussion of what has meant the M-15 movement in English. I just found today three analysis by activists of communist and anarchist ideas (i.e. with more clear ideas than most, and also with more experience of self-organization than most). These are:
On the Seville and Barcelona movements (two different articles published together) can be read at The Commune.
On the Madrid movement, an article can be read at Libcom.org. This one is the longest one and arguably the one with a more clear vision of the pros and cons, of what can be expected and what not yet, of this movement.