Never used a DAP collar, but have recently got a feline DAP diffuser for our cats. We have had quite a few upheavals at home recently (brother moving in, new boiler fitted meaning workmen here for 2 days) and our very neurotic tortie cat was making herself ill, she developed cystitis which is often caused by stress, wouldn't eat, fighting with her sister etc. I was very sceptical about the diffuser but have to say I have seen a big difference in her, even down to she and her sister sharing a bed.
Never used one perosnally, but was impressed by the fact that I have seen improvement on one of my client's dogs. She's got a wee chih that I couldn't get near the first time in...without extra hands! However second visit she was a wee doll. Much more relaxed. They say she's about a thousand times better at home, too.
i used a dap collar for the first time in october as my dog is very sensitive to loud noises. it seemed to make no difference at all and as halloween and bonfire night(s) were imminent i spoke to the vet and used 10 mg acp tablets instead . i have used a dap infuser previously and felt that did help a bit...hope this helps.....