Walt Disney World and
Disneyland are the ONLY places in the United States where it is socially acceptable to sport a fannie pack. You will in no way look "cool" but you won't be mocked or laughed at. Teenagers in your party will protest to your wearing of the fannie pack but other guests will understand and refrain from judgement.
There are absolutely NO exceptions to this rule.
Some healthcare professionals use them in the course of their jobs but in IMHO they look ridiculous.
I can't comment on the acceptability of fannie packs at other Disney parks, because I haven't visited.
But I can add that you will look totally ridiculous in all major cities in Europe, Mexico, and Canada rocking a fannie pack.
I sported a fannie pack for the first time in May, and found them to be perfect for Disney touring. Although, it is imperitive that the fannie pack be removed upon leaving Disney property or you will look like a dork at the airport.