A good documentary tries to uncover and show more than one side. So you have people here who don't agree with what Banksy is or was doing. The documentary leaves the interpretation of what is being said to the viewer. A very clever thing to do. Do you agree with the nay sayers or do you think Banksy is amazing/great and has something to say? Maybe it does lean a bit to the latter, but again it is refreshing to see they included other voices too.
I would agree that Banksy does good social commentary. Now whether Banksy is a he or she or even a group of people, we still don't know. And the documentary will not enlighten you in that respect. If this wet your appetite though, I can recommend "Through the gift shop" ... a different kind of "documentary" also with Banksy ... well sort of. Way more funny than this one for sure, but both work for what they are