JAXB is great if you have to code to some external XML spec defined as an XML schema (xsd).
For example, you have a trading application and you must report the trades to the Uber Lame Trade Reporting App and they've given you ultra.xsd to be getting on with. Use the $JAVA_HOME/bin/xjc compiler to turn the XML into a bunch of Java classes (e.g. UltraTrade).
Then you can just write a simple adapter layer to convert your trade objects to UltraTrades and use the JAXB to marshal the data across to Ultra-Corp. Much easier than messing about converting your trades into their XML format.
Where it all breaks down is when Ultra-Corp haven't actually obeyed their own spec, and the trade price which they have down as a xsd:float should actually be expressed as a double!
