He is kinda alright in a priest deck built to support him. If you can get him to a 2/4, he is relatively hard to remove. Its main problem is that the card is only fine early on against a rush deck, or overall okay against someone trying to play a 10-cost minion.
That said, I do not think that we currently have enough support to build him out of priest. Without the Velen's chosen, it is a lot harder to keep him in the game. I have tried him in a deck that also ran power word: tentacles, but he ended up underperforming. It especially hurts that we have an emperor giving discounts to 10 cost minions floating around. I think mana wraith could be good should we have another piece of the puzzle, either a second one at 3 mana that is a 2/4 out of the gates, or someone at even higher costs that taxes for 2. If we could find another good buff out there, then I think there could be a deck in there.
Back in the day I already played him to some success. I do not play enough to get him to legendary, but my mana wraith deck did crush until the season was over, at rank 7.