Beside from his slaying of Qui-Gon and his subsequent death at the hands of the young Obi-Wan, I don't know many things about Darth Maul.
I read somewhere that DukuDooku 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 Lightsaber skills, how was his Force mastery? Did he had enough potential to overthrow Sidious one day?