Since Ext forms live completely inside the browser's memory space there isn't a postback every time you click a button or expand a tree node like there is in ASP.NET. I think the delay from a postback makes the user experience feel choppy, especially if you don't have a fast internet connection. ASP.NET makes it very easy to hook into any DOM event, but since these event handlers live on the server, hooking into these events causes a postback which in turn causes the whole page to reload. Moving all this event handling logic to the browser makes the application seem a lot faster.
There's a list of controls on the left. You drag a control onto the form, re-size it, edit its properties and preview the whole form. It's very easy for laying out the form (especially if you're not familiar with Ext). You can even setup all the data sources (AJAX calls) for controls, like the grid or the tree, and then preview the form with real data. The major shortcoming is that it's only a UI designer - there is no integrated code editor. You have to export the project to add all the program logic and event handlers via another editor like Eclipse. On the other hand, using the designer in conjunction with another editor isn't particularly difficult if you have the designer project saved in the same folder as the rest of your application, its just a little painful to have to switch between applications, I suppose.