Originally Posted by Vhad

Edit:
Not entirely sure how it works in beta at the moment, but from the picture at blizzard's site its +80% mana cost -0.4 sec cast time stacks 5 times, if it keeps stacking like that rank 1 goes from 190 mana 2.5 sec cast to a 760 mana 0.5 sec cast spell - now granted 10% clearcast you would have to go through 7600 mana spamming max buffed arcane blast to get 1 clearcast. Its late and my math could be wrong, but that doesn't spell efficiency to me.