Damping Sphere has been insane thus far for me. . . in a Knight of the Reliquary/Thalia Shell, that is. As for folks saying they are trying it in a PO list, why? I use it in Knight Humans to BEAT PO decks so why on earth would THEY want to use it? It shuts off one of the most important cards in their deck in Tolarian Academy. I have a very spicy Knight brew with the card that anyone is welcome to ask me to PM to them, but I'd like to keep the tech under wraps until I crush at a tournament with it.
For the same reason Brian Kelly was running Chalice and 3-Ball in his PO decks... PO breaks the symmetry of the hate pieces as your opponent can't combo through them, but you can bounce them with your own PO and proceed to combo off. It hasn't done enough though.
If the card is only good in your fringe lists, it doesn't really justify the lofty expectations players have had and fears of a "Grafdigger's Cage" level of influence on the format. I thought this card would be a niche role-player in the format and it seems like that has been the case so far.