Aside from Frontier Airlines, I don't think there has been much I've gushed about more in the past year than Ext JS. I started using it when it was still called yui-ext, and even then it was going above and beyond what any of the other JavaScript libraries were doing. And it looked good. Really good.
What would you rather show a client, Dojo or Ext?
Dojo
Ext
You like tabs? Sure, who doesn't. Well Ext has scrolling tabs. One of the biggest complaints I've seen has been about the documentation, and the Ext team has listened and produced some great API documentation with an API browser that puts all of the other toolkits to shame. Seriously, if you're trying to sell a sexy UI toolkit, ugly, barebones HTML is not the way to go.
I've been showing Alex some of the ways I'm using Ext and while he likes it, he described it as almost being it's own language. In a way that's true, but I think that Ext 2.0 addresses quite a few of the complaints people had with 1.0 and 1.1. There's more consistency between components, and for the most part everything just makes more sense. Vague, I know, but if there really is less work and head-scratching with 2.0 to produce anything. Creating a Grid is no longer confusing and frustrating, the library of community-contributed extensions and themes is growing and the UI objects are all Components that support plug-ins. It still can extend YUI, Prototype/Scriptaculous, and jQuery, or you can use the native Ext library. In fact, what I really want to see next is a Jobs/Gigs board to make it easier to find Ext developers looking for contract work.
Read the Introduction, the Overview, browse the API and check out the rest of the Ext demos.