We finally started a somewhat meaty RIA-ish project at work. My original intent was to use ExtJS for the bling, but it wasn’t through the corporate approval process and the available themes didn’t match the desired corporate color scheme. So instead of ExtJS, we’re using JQuery UI 1.5.
So far, I’ve been extremely impressed. It does not have near the number of UI widgets that ExtJS has, but I haven’t needed them for this web project. One of the most impressive things about JQuery UI is how easy it is to theme. Some silly-smart folks over at Filament Group created an on-line theming engine for JQuery UI called ThemeRoller. With ThemeRoller, I can specify the RGB values to exactly match the desired corporate color scheme and even add some cool effects. There is also a great blog post on how to use the ThemeRoller theme to create your own components.
I still like ExtJS, but I think JQuery and JQuery UI are really starting to get a lot of traction. The ability to create custom tailored themes is pretty huge for corporate and custom development, and right now, JQuery has the leg up on ExtJS in this space.