Extjs Summary

Pros

• Commercial and Open Source licenses available
  
• High performance, customizable UI widgets
  
• Well designed, documented and extensible Component model.
  
• Good widgets. ExtJS is a superset of the widgets of all available JavaScript frameworks. With no obvious bugs it is great from a end user's perspective.
  
• Good API documentation. If you need to look up something you’ll probably find it.
  

Cons

• Applications do not degrade gracefully. Turn off JavaScript and you are left with a few lines on your screen.
  
• The CSS. It’s nearly thousand definitions which are giving you no idea which is used for what purpose.
  
• The HTML is huge. Many tags get so many CSS styles assigned that you don’t understand what’s going on. DIVs are nested so deeply that you’ll need a minute at least to find a certain widget in Firebug.
  
• Issues in loading the start page because of a 500 KB library
  
• Development/Customization  effort is significantly high in terms documentation effectiveness & IDE availability.
  

Cross channel Support

Extjs supports all major web browsers including IE 6+, Firefox 1.5+ (PC, MAC), Safari 3+, Opera 9+ (PC, MAC). Though there are issues when custom DIVs are added because parent child relationship does not render in the same way in all the browsers.

In case of an advent of a new browser one has to depend on the company Extjs as there is no way to customize the existing Extjs APIs.