ExtJS Cons:
Complex framework - not for the faint of heart :)
Steep learning curve
There are bugs - yes there are. However, most issues arise from not using the framework correctly and not understanding the documentation thoroughly enough.
Evolving framework - this can be viewed as both + and - but there has been ~7 releases this year.
ExtJS Pros:
Truly versatile client side MVC framework with the most complete set of components and widgets currently available.
Extensible. Literally Ext.extend() is the first thing you do.
Beautiful. Out of the box theme is so well designed that most devs with no theming skills ( we are not pointing any fingers) can create truly professional and good looking web apps.
Documentation is by far the best I have seen of any library, API, or open source project. Complete with API docs (even though some holes are there) with live code samples you can tweak and see what happens + source code. Great examples spanning most of the library. Excellent guides on most important topics from MVC to Theming. Even a video library. This documentation has been greatly updated recently so if you have not seen in the last 6 months - well, go look!
Forums are very much alive with many key experts actively participating on daily basis.
User Extensions (UX) - a large user community leads to an amazing array of creative solutions.
It's giant! It has so many great components that I have not had a need to truly extend something beyond recognition. Did I mention the learning curve?
MVC - for any self respecting RIA app MVC framework is a must
Built tools
Theming support with SASS and Compass
... there is much more.