I go with PI, but I haven't looked into it much.. at what point is the haste "not useful"?
At 50% haste in Shadowform, or in other words once you reach the GCD cap, Intellect starts outweighing Haste, slowly forcing you into gemming pure Int.
When you cried I'd wipe away all of your tears
When you'd scream I'd fight away all of your fears
And I held your hand through all of these years