I'd seriously consider using this, just because of the stagnancy of jQuery UI. It's a massive project with hundreds of long-open tickets (despite thousands of dollars spent on incentivizing developers over the summer through http://rewardjs.com/). 1.8 was released in March of 2010, and the last milestone release for 1.9 was back in May.
To be fair, a lot of jQuery UI's development headaches come from supporting IE6, while Kendo only touts its support for IE7+...
I would like to make a guess and say that the stagnancy of jQuery UI has a lot to do with jQuery Mobile (which if I'm not incorrect actually is a child-project of jQuery UI). I do however agree, jQuery UI is in many ways awesome, but needs to pick up speed.
To be fair, a lot of jQuery UI's development headaches come from supporting IE6, while Kendo only touts its support for IE7+...