Legendary items are technically always level 40 to equip, but the powers themselves level up as you going experience (which will make the experience gain on uniques at level 60 useful, as well as picking up orbs at level 60).
They each have 5 powers to unlock. It takes about 3-4 hours to unlock each power (less with synergies/boosts for exp).
Example:
Norn Stones is like this (in testing now):
Rank 1: +1 Durability
Rank 2: +1 Fighting
Rank 3: +1 Intelligence
Rank 4: +1 Strength
Rank 5: +1 All Skills
That's roughly the power level of a Legendary. Since they are a brand new slot and don't have to compete with anything else, a person will be strictly better if they have a Legendary item than not having it.
Note that most Legendary Items have a random roll associated with their affix, but the affix is always the same (except Norn Stones which has zero randomness).
There is a crafting recipe to restart your Legendary so you don't have to buy another one.
Some Legendaries have different powers with each rank. Gungnir:
Rank 1: Critical Hit Chance
Rank 2: Critical Damage
Rank 3: Critical Hit Chance
Rank 4: Critical Damage
Rank 5: Odin's Anger
Some Legendaries have the same power that unlocks at each rank and a super powerful rank 5 power. M'Kraan Crystal is:
Rank 1: Bonus Spirit
Rank 2: Bonus Spirit
Rank 3: Bonus Spirit
Rank 4: Bonus Spirit
Rank 5: Chance on hit to eliminate all enemies on screen to the White Hot Room.
There are Legendaries for every type of build in the game, including melee builds (Excalibur), AOE builds (Cask of Ancient Winters), slow builds (Also Cask), ranged builds, mental damage builds (The Warlock's Eye), pet builds, energy builds (Cosmic Control Rod) - as well as generically useful items that are useful for everyone.