I purchased one when I worked for Petco. I bought it for my 8-10 yr. old Pointer mix. She was a rescue, chained to a tree most of her life and used for breeding - has almost no teeth from gnawing on her chain. She is as sweet, gentle as they come, but suffers from anxiety. She doesn't practice any behaviors like pacing, never barks, etc. She just kind of sits there and shakes non stop some times then she is fine. Certain sounds make her hide under the bed overnight or hours at a time - thunderstorms, loud noises, the ice maker, etc.
The Thunder Shirt didn't do much, I could've saved $ because even if she's wearing her doggy hoodie it has the same effect and soothes her a little more than without. Another thing I do since she sleeps about 20 hours a day is wrap her with a throw blanket.