On 4th of July this year, two Westies wore it, at my recommendation. These siblings love to bark. They are not leash trained, so they pull like crazy. The Thundershirt descreased the barking by 98% - though they still went ballistic when a skateboarder sailed by, barking at other dogs ceased. They were a bit easier to walk on leash, and overall, calmer. I've witnessed this working successfully on about a half dozen animals, and don't know of any failures.
Now there's a Thundershirt for cats. Since my cat has a difficult time with the 4th (as most animals do), and he doesn't need further stress ( he was diagnosed with renal disease 2.5 years ago), I decided to try it. The effect it had on my kitty was nothing short of amazing. He became such a mellow fellow I was able to clip his front nails for the first time. He also became super affectionate and cuddly. I plan to put it on him the next time we go to the vet since he loathes traveling in his carrier.
The company offers a 45 day money back guarantee; Petco, 30 days. It comes in small, medium and large.