waresbear basically said it all.. responsive, chewing, giving, not showing any signs of aggression or pain (throwing head, chomping/gritting on the bit) and not trying to kill you... :)
I know my horse hates his new bridle.. because he always tries to flick his head away from me when I try to put it on.. and while I'm doing up the throat latch and whatnot. But he works well in it, he's responsive and doesn't act like he's in pain. He just doesn't like when I put it on. Eventually I'll get him a custom made one.
But same deal with bits.. especially since the wrong bit can cause a LOT of damage.