I wasn't saying that was a good play, just explaining how the card works.Or just play Forbidden Shaping and get a 4-cost minion, and save Knife Juggler for later.
My guess is that the card will be amazing at two times.
At 2 mana on curve. If you have nothing else it's worth using it. Especially against aggro and especially in Arena. Don't use it at 2 mana any other time.
At 6 mana almost anytime. Others will break down the average stats of minions and which mana costs have the best chances to make or break your turn, but my gut says 6 mana is where it's at. That is the cost where you don't see much stat/mana drop off and there are some really great cards with abilities or deathrattles.