By Sinespe (11,243 – 11·19·64) on 2009/11/19 (Patch 3.2.2)
This is insane for DoT classes, especially Shadow Priests, for the following reason. Pain & Suffering, when it refreshes SW: P, only resets the damage it does based off your spellpower value. In other words, if you have a trinket proc for an extra +590 spellpower, that will be applied to SW: P the next time you Mind Flay; and, when that proc fades, the next time after that you cast Mind Flay, the extra spellpower will be removed from SW: P's damage.
This, however, does not apply to any other damage-boosting stat. I.e., it does not apply to crit. If you were to use this trinket and then hard-refresh SW: P -- in other words, manually press the Shadow Word: Pain button again, rather than refreshing it through Mind Flay -- it would gain the 920 Crit Rating thanks to the buff, and that would remain for as long as that particular SW: P were kept on the target. If it is possible to keep that SW: P up for the remainder of the fight, it would maintain an inflated crit rate. At 45.91 Crit Rating required for 1% of Crit at level 80, that would equal a massive 20.04% continuous Crit chance boost on SW: P.