Added in that it has armor class penalty and Arcane Casting failure and that it is nearly as rigid as steel.
An average cloak would have 10 hardness and 15 hitpoints. Covering your body it would give you a +2 or +3 bonus possibly. You would have to ask your DM.
As i've written it, the AC is determined as if you were wearing a metallic version. So it would be the AC of a cloak completely made of steel. If that doesn't exist in the game then your DM would have to decide what it equals.