ExtJS is my personal favourite - amazing collection of ready-to-go components, very good doco, a strong community, loads of extensions etc. The performance is also very good.
There have been some issues regarding licensing with some OS purists feeling that Ext 'tricked' its early adopters. Personally I couldn't care less. I treat it as a commercial product for which i'm more than happy to pay $300.
One issue few mention about Ext, is that they are shipping bugged releases to non-subscribers.
You only get access to bug-fixes through their subscriber channels, which means that if you go on their site right now and download the public release, it has bugs, they know it, and they let you download it anyway. By my book, that breaks the software engineering code of ethics.
There have been some issues regarding licensing with some OS purists feeling that Ext 'tricked' its early adopters. Personally I couldn't care less. I treat it as a commercial product for which i'm more than happy to pay $300.