Being in the tail end of a big ExtJS development I would have to agree with fzammetti. The amount of custom component code I had to develop was tiny, with the built-in components delivering most of what I needed and the community extensions filling in the rest. I could focus squarely on writing business logic, without having to write boilerplate code and without having to constantly deal with cross-browser bugs (aside from the extra comma in IE issue, which I solved by adding syntax checking to my editor). That’s a really nice change of pace when it comes to client-side programming.

First, putting aside smoofles’s comment (which I disagree with, but it’s an opinion so it’s cool)… I think Ext provides the best-looking, professional and polished pure HTML/JS/CSS UI available today by default. I think there’s other libraries that *can* look better, but it requires tweaking or adding stuff, both of which you can do with Ext as well. Simply using the default Ext theme and widgets will give you a solid-looking UI automatically, which isn’t true of every toolkit out there.

Second, the way the library is structured and constructed I find to be superior to all the other alternatives, and in many cases by quite a bit. It’s an extremely logical, well thought out object-oriented model that goes along nicely with my Java-centric mental model. It just makes sense (99% of the time at least) and is built in the way you’d kind of expect it to be, which makes using it a lot easier and more intuitive than other libraries.

Third, it provides, out of the box, a collection of extremely powerful, robust and mature UI components, better than most other libraries IMO. It provides all of the basics, and then a lot more… there’s very little that I wish Ext had that it doesn’t, and most of the “missing” stuff is available as extensions (or you can write your own, which leads me to…)

Fourth, Ext is an extremely extensible framework, and more importantly, it is so in a very logical manner. Most good toolkits out there today can be extended, but I find the mechanisms Ext provides to be far more logical. This is directly because of its highly object-oriented design, which sets it apart from most other products in this space.

Fifth, the stuff Ext provides aside from the UI widgets is top-notch, and I think frankly that this gets ignored all too often. In my mind, there’s really only Dojo and Ext when it comes to providing *everything* you need to build modern RIAs. None of the others is as far-reaching as those two (without extensions I mean), and between them I’ve simply had much better experiences with Ext (I don’t mean that as a swipe against Dojo, just expressing how much better I find Ext). There’s all sorts of handy utilities, classes and JS extensions in Ext that just makes life so much better for us code monkeys.

Sixth, the way Ext influences your coding style is, to me, in a very good direction. If you at least roughly do things the “Ext way”, you wind up with an architecture that is clean, simple and robust. What’s better is that the “Ext way” isn’t much different than how you’d write robust applications with other technologies anyway, but that’s something that hasn’t always been the case with JavaScript in general… that Ext sort of pushes you in that direction is, in my experience, a very big positive, particularly when dealing with less experienced developers.

Now, to be fair, there are warts with Ext just like with any other product, but that’s for another time :) I think the positives far outweigh the negatives in any case.