When the jQuery UI Button widget was introduced, I was amused, but I wasn't exactly blown-away. It's not as exciting as Autocomplete, Accordion, or some of the other widgets, but handy nonetheless. However, when I discovered that it supported checkboxes and radiobuttons, I started to get excited, but only because I had yet to learn that the idea was better in theory than in practice.
The state difference between 'on' and 'off' are almost impossible to differentiate on some of the themes, leaving the user unable to discern whether or not his/her checkbox is 'on' or 'off'. The problem isn't as drastic for a radiobutton, since the user can compare with the other radiobuttons in a different state, but it still wasn't practical.
But, there was hope. Having a good foundation, I knew that with a few slight modifications, the Button widget's potential could be fully realized.