Well I know it's anecdotal evidence at best, but just now I got a Warrior arena draft with both the Black Knight and Ragnaros and it's hilarious how much they can get done just on their own. Earth Elemental? Well that's cute, let me just slice it up like it's nothing. Oh, you dare trade with me going into turn 8? How do you enjoy an 8/8 with charge to the face?
I know arena isn't too balanced but the impact legendaries can have there is downright sad/hilarious dependding on whether you have them or are on the receiving end of one.