I've heard a lot of people say how great they are, but I could never use them. Of course, I also mainly stick to unique "hero" characters and the occasional created (thieves, in 2 and 3, seem to make better gunners than, well, gunners.).
It's sad, because I truly like the magic knight character design.
It's not the actual magic knight that does the damage, in my experience. You have them cast one of their element-boosting spells on another of your characters. Greater affinity for an element = greater damage with attacks of that element. Somewhat useful since Adell has a few fire-based attacks and Yukimaru uses water.
Anyway, magic knights always were one of the more attractively-designed classes, in my opinion. Nice to see some art floating around.