Ext.JS Review by ajaxBlender
Ext.JS has a carefully thought out API and components. It is well-documented and their site has very good demos available. We started working with Ext.JS from its first release and now have several years of experience developing on its platform. We collectively can say that you probably will not find a more powerful framework for web application development (at least currently.)
The only disadvantage is that Ext.JS is a little to heave for use on websites. For this reason, they have created Ext.Core which is intended for websites development only. It is the right path to go, but from our point of view Ext.Core is a too young of a framework to use for mission critical applications.
Therefore if you are going to develop an advanced web application which requires a rich UI, you may want to consider Ext.JS. Otherwise, if you are looking for something just for medium complexity website, you should looking into other frameworks, such as jQuery, Mootools or script.aculo.us.