I'm really enjoying Arch-Thief Rafaam in my Renolock, games are usually very long and I've been able to play him quite a lot, and he has won me many games with his artifact. I've used all 3 at different times depending on the board.
Lantern of Power: Give a minion 10/10
This has won me the most games, it's useful if you have board control with a few creatures and are almost guaranteed to be able to go face with one of them.
Mirror of Doom: Fill your board with 3/3 zombies
This has also won me a few games, it's very useful if you don't have board control or have just a few creatures. A few classes can easily clear them, but since it's played late in the game. If you know they played their board clear, this is really good.
Time Peice of Horror: Deal 10 damage randomly to all enemies.
Probably the most useless one, the random aspect makes it less reliable than the other two. I think I used only once in a ver specific situation where my opponent had only 2 life left and board control with taunts, and had Lethal on his next turn. All I needed was 2 damage to go face for the win which I got.
Overall, it's almost always useful...and very often end up being lethal the next turn with the Artifacts