I would never be caught wearing one in my hometown, but in DL, it is a must!!
In December, I wore a big honking one with 3 pockets.
In it:
A water bottle, gum, fruit leather, advil, hand sanitizer, digital camera, batteries, map, bandaids, money and passes.
Hubby wore one too. In his was mints, the video camera.
I don't like the idea of having a big backpack on that you have to take off on all the rides. The belly bag can stay on all the time. On the rides that have lap bars, I just turn it to the side. It is also much easier on your back.
It also keeps the cameras etc.. dry on the water rides.
They do a quick check at security.
I wouldn't do Disney without one!!