What all does this book cover? Sun has said that there is going to be a whole range of Java - FX related stuff going to be out in the market. So what all can we know out of this book. Does your book require any prerequisite other than Java/J2EE?
JavaFX can do everything that Java proper can do, including communicating asynchronously with a server. If that's what you mean by AJAX, then yes, you can use a JavaFX GUI to replace your AJAX web pages. The drawbacks will be the same that keep people from using Java web GUIs (applets) today. The Java 6 Update N (a.k.a Consumer JRE) is meant to address some of those shortcomings, though.