star wars - How powerful was Darth Maul?

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 Dooku 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?

Answer

Darth Maul did possess some significant levels of Force mastery, but according to Wookieepedia was inferior to "true" Sith Lords:

in order to avoid breaking the Rule of Two, Sidious was only allowed to train the young Zabrak as a weapon; he was never trained to be a true Sith Lord in the same way as Plagueis and Sidious.

While his greatest strengths were his abilities with weapons (Maul was a master of the fighting styles Juyo, Jar'Kai, and Teräs Käsi, with training in Niman, and mastery of the lanvarok), Darth Maul possessed a number of notable Force masteries:

Maul displayed a reasonable amount of competence in telekinesis, utilizing Force grip and Force choke while interrogating Hath Monchar, and being able to activate a set of door controls behind him with a flung piece of wreckage.

While dueling Obi-Wan Kenobi, Maul used both Force push and Force lightning against the Jedi (this reference is from The Phantom Menace video game, which I believe makes it C-canon).

Maul also demonstrated a resistance to Force lightning, shrugging off a barrage unleashed by the Nightsister Mighella while hunting for Alexi Garyn.

Darth Maul was skilled in combining Force talents with mechanics. Sidious gave Maul the funds and schematics he needed, and provided his apprentice with a secret facility on Coruscant in which to work. There Maul constructed his signature speeder bike Bloodfin and the Dark Eye seeker droids. Using the Force mechanical skill mechu-deru, Maul modified the protocol droid C-3PX into an asssassin droid to guard his ship.

Also, while fighting the Force-sensitive Drovian Silus, Maul used the Force to overload a nullifier field that had been preventing him from using his lightsaber.