I found it pretty easy to build Web pages using the technique. Anyone who is new to AJAX will discover it's much more convenient to let Jaxer handle all of the background work of bundling and unbundling the data. It's all mostly automatic and even simpler to use than some of the AJAX libraries such as jQuery.
It's not clear how much support Jaxer is enjoying these days. The server used to be bundled with Aptana's other offerings, but now it's left alone in a corner. My guess is that many people don't need that much help handling AJAX calls now that libraries like jQuery simplify the process.
The ideal job for Jaxer will be one where most of the work is done on the client but some crucial part must run on the server. It's very easy to make code run on the server, but it's not so easy to write complex server-side code. There are plenty of jobs like this, and the people coding these jobs are the ones that Aptana is targeting when it says you can write an entire Web application in one file.