As both AST and WHM, I will never use PIE again. 99% of the fights requiered in this game leave me with over 7k HP - including massive dps'ing - (from a total of 13k).
The only 1% left are the savage raids, which would burn my PIE out. I compensate with food.
As an AST, when I get the chance for an optimal BiS, I'll probably focus on getting DET/CRIT
For my WHM, I'll always take Spellspeed over DET/CRIT.