The only thing Firebat said was "Cho'Gall works great with Siphon Soul", which is kind of a "well duh" moment on its own. But that's not enough to make the card top tier. You have to consider that you have to have Siphon Soul in your hand, and whether or not that synergy with Siphon Soul alone is enough. Like I said, I could maybe see it in a deck that runs both Siphon Soul and Shadowflame and can afford to double up on at least one of those. That completely excludes Renolock and doesn't even include a bunch of Handlock decks.
That is the thing. The effect is very powerful, but, besides Syphon Soul and Shadow Flame, there are few spells that can be abused. Maybe Sense Demons and Bane of Doom as well. Rafaam could be a possibility. Either way, casting spells for 0 cost is insane, lets see how it turns out later.