At the recent Usability Week put on by NN/g in Seattle in November 2012, this subject came up. They said that their research showed that hardly any users increase or decrease font size by any method other than native browser zoom functions.
They said in their studies it was very clear that users who have trouble reading small text are very used to using their browser's native function, so that is their default action.
Additionally, they said that there is no standard convention for placement and design of font size increase/decrease within the page itself. For placement, I agree. Sometimes it is within the page (maybe near a byline for a news article). Sometimes it is in the utility bar at the top. Users are not conditioned to know exactly where to find the feature and they don't know exactly what it will look like.
In contrast, they know how their browser's native functions work. It works the same, it looks the same, and it is located in the same place for all sites because it is a function of the browser not the site.
Of course all of these studies are behind a paywall so if you really want to know their details I'm sure they'd be happy to sell you a copy of their research.
However even on your own, try adding some Google Analytics event tracking to the controls in your page if you already have them and see if they get used.
My recommendation, however, would be to spend time making sure that your site scales well under magnification instead.
Update: this is a free report from NN/g on usability guidelines for the web. I haven't read every page yet but it doesn't seem to address the specific topic of using in-browser text resize feature.
However it does provide a great insight into HOW people using technology like screen readers or magnifiers use a page and what impact that has on the design.
There are several examples that support my original recommendation, which is to spend more time making sure your site scales well under mangification/zoom. For example:
Participants in our study told us they cannot magnify some sites with their screen
magnification software. When this happens, they usually just leave the site. Some
users said they’d call a sighted friend to help them use the site. Regardless, they
could not use the site on their own at all, and were frustrated by this. In the tests in
Japan, one participant showed us a website that he could not magnify. The site
designer specified the fonts in pixels, in the style sheet, making magnification and
normal font enlargement impossible.
On the subject of whether a user with vision problems will "know" how to use tools other than in-page controls that you add:
If you have never seen a screen magnifier, try to find and use one. As you’ll see,
the results are very different than simply increasing monitor resolution or selecting
the largest text in your browser. Users working with screen magnifiers make text
big—really big. To see an entire page, they might need to scroll horizontally and
vertically many times. The bigger the page, the longer it will take them to see the
whole thing, and the more difficult it will be for them to remember everything on it.
Our participants were very adept at using their screen magnifiers. They knew how to
zoom, change contrast, filter colors, and most importantly, scroll vertically and
Some users chose to simply highlight text, or chose select all, because highlighted
text appeared as white text on a dark blue background.
The key take away here is that if someone truly has vision problems, they are going to know how to use tools that are much more helpful to them than any widget you add to the page. How your page looks when using these tools however does take some planning on your end.
Here is a video that illustrates what one of these screen magnifiers looks like and how they are used.