I used one the first month after moving. Neko-chan settled in within 12hours. However, I can't be sure if it's because of the Feliway or her own personality. Neko-chan is a rather confident kitty.
I have used one for Artie... and again for Geoffrey. I think they do work. I know Geoffrey is very scent sensitive. He is also very anxious. I can tell when they run out from his behavior.
Sometimes I just use them when I feel he is getting overly anxious about things.
I’ve used both the Classic and Multicat Feliway diffusers with a lot of success. And the Feliway spray has worked to keep Mocha from sharpening her claws where she shouldn’t. But some cats are simply not affected by it at all, much like some cats don't react to catnip. Unfortunately, the only way to know is to try it!