I've seen some pretty awesome power turns in 2s using onus...4x onus in 1 turn destroying a bunch of 80+ nora champs. After that, it was only a few turns to mop up the rest and destroy the avatar.
It's very niche. Very very niche. It's somewhere between the best and worst spells in the game, I know that much.
If you run Onus, you also have to run Ritual of Binding, to prevent Fortification and Transfiguring. Pretty much the only scenario where it works (ish)* is when you can move in close to the foe's shrine and he/she doesn't have many champs out anymore; however, why not just run a Vertical Push, or a Slaver's Whip for Possess 2, or, hell, even a Rock Trap to buy you time, right? *Zeventrech BGs sometimes run Onus, since, with him, you can easily end a match in a few power rounds with the right setup.
I ran a fairly successful Onus deck a while back, when Clops Hurler still had Create Chasm. Without the cliffing shenanigans, the deck is quite a bit slower than I'd like, but being able to consistently boardwipe almost every game was so satisfying.
