The effect will be revealed as soon as it gets drawn into your hand. Shadow visions will probably not reveal the effect and probably generate another effect as the original in your deck. Just my guess though.
Zoo priest is already surprisingly strong, this might push it over the top, depending on the effect.
Yeah, I play a zoo dragon priest now (only synergy card is 5/6, plus all the good cheap dragons), and it wins pretty often, and since it runs the 3/3 card steal and the 4/3 card steal it has value for days while still having amazing tempo.
Personally dont think its that great because he effect is random you cant play around the effect to insure it has enough umph to actually make it a playable card while the card is good its effect is so random that without being able to play around the effect its at best good for arena i dont think its good enough to see play main stage in priest currently over other cards for just 3 mana. If the effect was a choice then it would be much stronger. But a random effect i just dont see it having enough consistant value though maybe im wrong I dont see it currently being that good.