Mustang tackles with Turkey's most important problem, the issue of woman in society: her secondary place and the patriarchal chains around her, due to culture, politics, religion and socio-economic development. Therefore, the director's effort should be viewed as most welcome and needed attempt. I think the director manages to show the virtual prison experienced by all sort of women in our society, through the case of those sisters.
However, and despite the director's best intention, movie in general fails to deliver an authentic picture of everyday life and details in the country. Oppression of girls and their youthful desires is correctly depicted as a whole, but done in somewhat unrealistic ways. You get the feeling that the director, the script writer, and even the actors remain quite "foreigners" to the situations they are interested. Some are accusing the director as orientalist (playing to Western gaze) because of these failings. I don't agree, but it would be better if they had worked more on the everyday life and details of the countryside where the film occurs (like better local dialogue, better acquaintance of local customs).
In short, I found Mustang a very important movie, but with its own problems. Technically it does not offer something novel, but it is courageous directly to point the gender issue in a very conservative society.