Click Density recently posted the results of a survey of browser text size settings across the 1,500 sites they currently track.
For consistency’s sake their analysis was limited to Internet Explorer users, but the results are pretty conclusive: 99.7% of web site visitors used the medium (default) text size.
This doesn’t even vary much depending on screen resolution either.
I find the ability to change my browser’s text size very useful (I just did it in order to quickly re-read this post).
It’s a shame so many people are apparently unaware this ability even exists (I’m assuming that they’re unaware rather than that they simply choose not to use this feature).
I don’t see why browser makers can’t display this functionality in a more obvious fashion — either within the default set of browser buttons or in the browser toolbar.