I tried the Sentry calming collar. It did seem to help, although I didn't put it on the cat; I put it in his stroller with him and it seemed to help him get used to traveling.
You are going to get many contradicting answers to this, eventually. Calming collars seem to work VERY well on some cats, not at all on others, and you get different reactions with different brands, as well. The best anyone can say is, "Try several brands on your cat, and see if something does the job for you." I wish I could give a better answer, but...it just depends on your cat, and which collar.