The one mexican black I've had was nasty; it was his life's goal to bite and try to eat everything it seemed. Kings in general I find to be pretty high strung and quick to bite and seem to have off days a little more often then other snakes; not to mention that feeding response. My baby kings have all been very musky, my adults are handleable, I think conditioning them to handling helps a lot with many of them. I still say though that all kings are crazy to one degree or another.
Milks on the other hand are usually pretty shy and jumpy, I've rarely had any try to bite though, usually just musk; I usually find them a little more handleable compared to the kings I've had, and a little more pleasant to deal with.