I agonised over this one for ages for a site I worked on. There's a huge part of me that echoes Dan Diplo's feelings on the matter: it's built-in browser functionality, it doesn't need replicating in a subtly different way by every individual site on the www. If you add a font-sizing widget to your web site, you might as well also add 'print this page', 'go back', and 'bookmark this page' links all over the place.
BUT, people argued that other people didn't know how to resize the text in their browser. A lot of people don't even know that it's possible.
I started reading up on the matter, and thinking about it a little less passionately. In particular, I came across this question on uxexchange and the top-rated answer there just about had me convinced. I added the widget, in such a way that it's reasonably obvious without wasting valuable screen estate. I haven't looked back.
Other than the argument on the uxexchange post, my main reason for implementing the widget was the ignorance of font-sizing options in the browser. I wish everyone was more aware of that feature, but wishing it doesn't make it happen. I think it's a real shame that most of the browsers removed the prominent font sizing icons that used to be on the main toolbar - you have to go back as far as IE3 (!) to see this excellent feature (in that particular browser, at least). It's far more hidden than print, back, and bookmark and - mainly for that reason - I think the font-resizer widget is, on balance, justified.