Normally this would be alright if it wasn't a Priest card, since they can already heal themselfs with their Hero Power, so they have no need to play a Lifesteal minion. Also Earthen Ring Farseer is better, especially since it can heal minions.
The nice thing about this as compared to Earthen Ring Farseer is that aggro MUST trade into it rather than risk letting it attack multiple times. Whereas if you play Earthen Ring Farseer against, say, Pirate Warrior, he'll simply ignore the creature and go face most of the time. This buys you not only the 3 health, but also an attack from an enemy weapon or minion.
I'm not going to completely dismiss Lifesteal. It seems to be the best keyword since Taunt to actually stop aggro decks in their tracks.
My main concern with this particular minion is it's not likely to trigger more than once, making it a more niche Earthen Ring Farseer. I guess they're both about equivalent in that one can be a targeted heal that affects both minions or your hero and the other is hero only but has the potential to trigger any number of times between 0 and up. Neither is inherently better or worse than the other, just different.
The real question is, is a minion that's 'about' the same power as Earthen Ring Farseer good enough as a class minion to see constructed play? With enough support, yes. The new priest 2 drop is a good example of the kind of support this card needs. By itself though? No, I don't think so and I'm unconvinced that with the knowledge we currently have that enough support exists for this card to make it's way into your deck.
For the card to be good enough, we need to have it deal damage more than once and unfortunately it's just too fragile to expect that. Sure, PW: Shield, Kabal Talonpriests, even Divine Spirit make this minion more threatening. But those cards also made Shadowfiend more threatening and we know about how much play that saw. The only real difference between the two is you can usually expect Acolyte of Agonyto trigger at least once where Shadowfiend wasn't even that reliable.