(Summary and picture from Official Elder Scrolls Site)
At very high levels almost no locks will be average. The once a day power is extremely weak. However, at low levels, it can be very useful when low on lockpicks or doing quests such as Unfriendly Competition which require to open two average locks.
There is also a Tower Stone that grants two powers, the Hand of the Master (+20 armorer for 120 seconds), and the Warden Key power, which can open every lock of "Hard" lock level within 15 feet of the target, once per day. The stone is located just Southwest of the Imperial City Waterfront , on the tip of the small island across from the Waterfront docks.