Power Critical is a strange feat. Basically, it changes your crit range to 2-20, but only for one attack. This means that it is far more powerful for attacks with a big crit multiplier than for attacks with a big crit range.
The only character I've seen take it so far is the 1/2 Orc Paladin with the Greataxe, who uses it in concert with Bless Weapon (no need to confirm), Power Attack (+3 damage per -1/hit is nice) and Smite Evil. She can, at 21st level, reliably pull off something like 6d8+2d6+150 damage with a very good chance to hit. Sure, it's only once a day, but with damage like that, it's great against boss monsters.
I'd also consider taking it for mounted characters (x5 or x6 damage on a critical), or anyone who can pull off very high damage attacks a limited number of times a day, like Clerics with the Destruction or Strength domain.
Basically, I've disallowed it in my game, as I find it too good for certain types of characters, and useless for others.
--Seule