The Mirror-Klingons and Mirror-Cardassians didn't really seem that much different from their counterparts in attitude, though Worf's a possible exception.
I've got the worrying feeling that the Mirror universe is actually "our" reality, and the one we typically see in Trek is the "alternate"...
I'd say it depends on the characters. Mirror Spock was always very similar in many respects to his counterpart (and developed a secret jealousy of him according to the Mirror Universe Saga, because our Spock was able to pursue Vulcan interests and have genuine friends), and the MUS also depicted Mirror Marlena as having changed as a result of meeting our Kirk. Mirror David, who was alive and well in the other universe, seemed to be very much like his counterpart and was definitely a hero.