Obviously, a 7-mana 5/5 with no immediate board impact is very bad.
But being able to play the effect twice, as well as the Horror's effect twice, and generate another horror, is damn good.
Shaman still kind of has the issue of "what am I doing with all this incremental value?" Maybe the metagame will change enough such that this is actually a viable strategy.
...that said, if it does become a viable strategy, you're going to be very hard pressed to find a deck that does it better than Shaman. And we've seen that metagame before. As the classes move away from infinite value, Hagatha (Hero) and Shudderwock are really close to that.