I use it a lot.
When they were working on the road, Steffie got very stressed, she even peed in the house. I got the plug-in and it really helped her relax.
When Precious, a very dominant rescue cat, temporarily moved in with us, the plug-in helped so much we decided she might as well live out her last years with us, and she did, with no problems at all.
I even use it when the cats are too wild when there is a storm brewing. There often seems to be a storm inside the cats, too, when the barometer falls.