Yes. And yes, that's how it works.
But it's not only useful against massive AC enemies. As Master Changer said, the second attack has a lower AB than the first, and so on. You may also be using a mode that reduces you AB, like Improved Power Attack. Or have a high crit range, but not a high AB...
In general, it's a useful feat if you have a good critical threat range and decent crittable damage (in other words, for STR based characters usually). Many times there are more important things to take, though, but if you have everything you need, it's a nice feat to have in those cases. Obviously, its usefulness also depends on how much crit immune enemies you're going to find.
Also, keep in mind that Improved Critical can be gained by other means (keen weapon, spells), but Power Critical can't.
Modifié par Arkalezth, 18 janvier 2012 - 02:54 .