What do you mean, corporate? I've used it, and it does incredibly well for performance and stabilty, has a beautifully consistent API, and the best documentation for such a big js lib. Give it a go, man, you won't be disappointed (I hope).
When you finally figure out where the API and demos are, it still takes quite a while to figure out if some jslib things are supported -- for example, some kind of rich text editor. I just looked again, and I don't think it is, but really I'm not sure.
I very well may try it, though. I've been looking for a project that makes a beautiful, cohesive interface, largely for web game programming purposes. I'm glad to hear that, despite its homepage, developers find it to be a nice project to use =)
I've been developing with it for a couple of years and am quite happy with performance. Of course its going to be sluggish if you're targeting ie6 or something since the DOM the framework creates is HUGE.
Would be nice if they ditched support for ie6 which would probably help a lot with DOM bloat.
In reality the Web UI was quite complex.
IE6 was in the original requirements and was ditched because it was unusable (luckily the customer agreed, were would have been in a really bad position if not).
IE7 has performance issues as well, even after many resources thrown at it.
I wasn't involved directly, so i can't tell you technical reasons, but i know the WebUI was "state of the art", Ajax, etc. The generated HTML code was not only huge.. it was extreme ;)
Also some JS functions were quite slow and adopting the latest ExtJS release also not possible because of not backward compatible changes.
All in all, everyone thought in the beginning ExtJS offers probably the most (featurewise). In the end, many are thinking of either going more lowlevel (jquery) or more highlevel (GWT). This will be evaluated.
From my experience with ExtJS and IE, the problem with IE is that it lazily evaluates stuff. Your ExtJS will load lightning fast in IE6, but when you start click on stuff it then runs the JavaScript to generate the HTML. Other browsers don't do this (or their JavaScript engines are fast enough to compensate). This is all a bit fuzzy. Haven't touch it in months, so I could be wrong or things could have changed.
The documentation leaves something to be desired. Working with ExtJS heavily for over a year, I found myself digging through the source code far too often.
I was continually frustrated until I finally settled down with a glass of hot chocolate, some romantic mood lighting, and started getting... intimate... with the source code. Even then, with GXT, it would be some nasty fights with a mediating debugger when it would stop talking to me and I had to pry out the reasons why. Usually, it was because some event wasn't being listened to.
A bad library is much like a bad relationship. When it's good, it's really good, but you forget that far too much.
