I believe they actually make a variation specifially for cats. So its possible.

 

DAP diffuser worked wonders for my cat in a matter of hours as well.

I really think a LOT of behavior is neuro-biochemical in origin, and clearly these pheromones, or what ever is in the diffuser, have a big influence on the brain.

 

I've heard wonderful things about them, but have never had them work for me. Our male cat decided he didn't like the addition of our kittens to the home and began to mark everywhere. Somebody actually sprayed right next to it about a day after I plugged it in... Sigh.
I suppose it did calm the girls down a little, as we no longer have kitten races at ungodly hours in the morning, but that was about all I noticed.

If you don't mind my asking, why did you purchase it? If it was a behavioral issue with one or two cats, they do make calming collars for individual behavior treatment. That may solve the problem of your kitty's personality switch.