I have found the response seems to vary quite a bit. I have 3 cats at the moment, two that are super sweet: one gets sweeter on catnip, while the other just gets nasty and turns into a biter (he gets the same way when he gets a certain pain med from the vet; they said both trigger the same part of the brain). Needless to say, he is banned from that pain med and catnip. My third cat is on Prozac for aggression, and he gets even more aggressive when exposed to catnip. I have two cats who have passed on, one who also became sweeter when exposed to catnip and one who had no reaction whatsoever.
What have been your experiences?