I don't know of anyone who has used a collar, but know of quite a few people who have used DAP deffusers in the home, some say they were wonderful, some said they made no difference, but no-one has ever said they made the dog worse.
What works for one dog doesn't always work for another, also owner expectations need to be taken in to account. See if your vet hires them, some do, then you could try before you buy.