So I've only used this ability twice so far playing on my WHM Alt, but I can say it's very potent.
The first time I used it, I poisona'd a NIN tank at Pascerpot, and about 3-4 Poison Breaths after that, the effect of poison from each was resisted.
The second time using it was on Wherwetrice with Stona, and the same thing happened. It tried using Baleful Gaze and the Petrify effect was resisted about 2 times.
The recast of it paired with the duration of resisting really makes it awesome, I'd like to try this on some tougher NMs in the future, It's definitely useful.