Conclusion
By now you should have a rather thorough understanding on how you can use the YUI Library in a Grok project. The main focus of YUI is not AJAX itself, but providing user interface components that can be easily reused and that work on all browsers. It uses AJAX for some of its functionality, but it is only a small fraction of it.
There are other packages out there (JQuery is very popular one for instance) that perhaps are better suited for pure AJAX applications, but if you understand how to use YUI, you won't have a hard time incorporating other libraries into your projects.
Source code for the final project can be downloaded from http://github.com/jmichiel/YUIdemo

