It works for me, with Galeon (basically the Mozilla browser) on Linux.
It should work on most sites. It possible that a site can specify font sizes using CSS in such a way that you can't override it, or renders the text as graphics (ugh! how brain dead is that?)

It's not just a mouse feature - it has to be supported in all applications that use it, but any application that supports some kind of "zoom" view menu item should work - I think the mouse driver sends "zoom in" and "zoom out" messages to the application.

Anahata