ExtJS is just, ...how can i call it, ..HUGE!!!, which means taking time when loading your ext applications and in the other hand ExtJS has everything you need to find in the programming bucket: window, trees, toolbars, grids, forms, etc., etc. manipulation. It is easy to create even an online desktop.
These whole bunch of "outofthebox tools" is just amazing for starting to build our applications and get results real fast and even faster if you are a freak at Java because extjs makes things programmatic and Java-esque.
If you are a programmer who likes to "personalize" and "control" your apps in excess and when installing your phpBB Forum, or your blog, you needed to customize it and getting control of the code then go with jquery, mootools, Dojo or prototype, but if you feel confortable with a the neat way of how extJS behaves in each of its controls then use it without fear!!.