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+...