The fighter and the cleric in my group use these. Basically they're the same as saying "I expertise for two" every single attack, except you don't need to attack to do it. So our half-dragon cleric loves it, to the point of taking proficiency in it. I (the fighter) took it because I really need the AC, (hardcore GM and an evil party means that you really don't want to go down in combat, you don't know if you're gonna wake up again, even if you stabilise) and since the other members of the party are good at dealing damage, I don't feel the -2 to attacks is such a big deal.
That said, I did swap it out after using it for 15 levels when I finally took Combat Expertise. -10 armour check penalty is HARSH when you're trying to swim.