I've recently resurrected a Brownie Six-20 Model C and successfully used 120 roll film through it.
The diameter of the spool shaft and spool ends for the Brownie are smaller than current plastic 120 spools, so I re-rolled the film onto the Brownie metal spools. You need 2 of these, 1 for the film itself and 1 for the film takeup as you wind on.
You'll need a changing bag or other totally light-tight space to do the re-rolling in; it's a bit time-consuming as 120 film seems to go forever as your winding onto a thin spool!
I've read of some people being able to trim or modify the spool ends of 120 film to make it fit, but this didn't look like it would work for my model.
Was it worth the hassle? You bet; it's very liberating peering down through a basic mirror finder & trying to frame correctly (harder than framing through a waist level Mamiya finder as it's pretty small), pressing the one and only button and hearing the dinky little click. No bells, no whistles, no dials, no overrides.
Now once I'd developed the negs I understood why it was sold as a popularist snaphot camera; the verticals are distorted, it doesn't do close-ups at all well etc etc but it lends a low-fi charm to even basic images.
I'll definitely be using it again.
There is a good pictorial about re-rolling film and other resources (including links to images taken on various Brownies) on this site
http://www.brownie-camera.com/
Edit: I see this has already been linked above.