Its very good, which is annoying because other classes gets conditional legendaries that aren't always that good when you consider the stars have to align for the comments heralding them as the new born sun kings! :L
This card is amazing because if you can stick it out you get tremendous value! - or you get wiped, because you built control shaman wrong!
Its 5 mana fora 5/5 with taunt, big whoop
However when it dies because your opponent trades into it rather than going face or for your turn 4 drops you shuffle in a stupidly powerful card.
Granted its "shuffle in" a stupidly powerful card, but its always a threat and your opponent becomes on edge every time you draw a card.
Control shaman is going to be insane because it will be able to handle aggro and its going to drop alot of stuff other control decks cant deal with.
Tempo mage for an example (will probably evolve into a highlander tempo hybrid) is the best case archetype for mages and they want to prolong the game, which means you'll draw the storm guardian [mages struggle against big health minions because their only wipe is via dmg or polymorph realistically ran in mage decks].
Priest and pally wont struggle but their meta is going to change so it wont matter :L.
Control warrior will probably just stagnate like it always has.
Considering some shifts in the meta realistically we'll get tempo-hybrid mage, midrange paladin, tempo-dragon-priest, aggro hunter (as always), demon lock (might return), token druid, mid-range/control/aggro shaman, control warrior (surprise).