Beside from is slaying of Qui-Gon and is subsequent death in the hand of the young Obi-Wan, I don't know many things about Darth Maul.

I read somewhere that Duku was considering him merely a beast, but I suspect his opinion of being distorted, as discounting his predecessor is a way for him to increase his value.

So, How powerful was he? Aside from his Lightsabes skills, how was his force mastery? Did he had enough potential to overthrow Sidious one day?