This is just my personal opinion. A developer preview is not good for learning an API. The API is not stable and you should still expect changes. For evaluating GXT 3 you should wait definitely until GXT 3 reaches a beta state.
We are still working on creating more javadocs and also the manual. You should expect much better javadocs once in the beta and rc cycles.
The most recent introduction documents I found (2/2013) are from GXT 2.x, and they are incompatible with 3.x right off the bat.
In my evaluation work comparing various AJAX frameworks this will score pretty low...
While there are requests here and there for a 'getting started', for the most part, the same rules that describe creating and manipulating widgets. As of GXT 3 (and especially with GWT 2.5), UiBinder works exactly the same with GXT widgets as with GWT widgets - other supported features include the Cell API and the GWT Editor framework - any existing documentation that applies to them also applies to GXT.
Javadocs should be much more complete than in 2011 when this thread was started, and the examples at http://sencha.com/examples are much more fleshed out.
The basic process for setting up a GXT project is pretty straight-forward: set up a GWT project in your IDE of choice, download GXT and follow the instructions in setup.txt. This is 4 main steps - add the jar to your classpath, add an inherits statement to your module, ensure you are using a strict doctype, and add a reference to reset.css to your html page.
Are you looking for hello world examples, or specific tutorials?
Yes while the widget examples are nice, what is lacking is an example putting it all together in a non-trivial example app. As it stands now the cheesey StockWatcher example provided on the GWT site is superior to anything available now for GXT as a getting started guide.