Java has a standard API for extensions that do XML binding, and there's
a Sun reference implementation.  

http://java.sun.com/webservices/jaxb/index.jsp

I use Castor for this, though not a standard JAXB implementation, and I
find it to be excellent and pretty efficient.  Many of my applications
are built starting with an XML Schema and I use Castor to automatically
generate objects from that.  Starting there, it's extremely easy to
Marshall/Unmarshall objects to and from XML, and I also do database
binding to the same objects.

The problem with having a "simple" ability to marshal or unmarshal, is
that the relationship between an object and its XML representation is
not always quite so simple.

If you don't really care about the structure of the XML and don't need
precise control over naming and type conversion, you can use XStream
very simply.

I use Hibernate to map my objects (the same ones I map to XML Schema) to
a database.  It's pretty efficient both in terms of storage size and
performance, but that's relative, of course.  Since I specialize in a
certain type business software, I consider the benefits to data
integrity and the design-by-contract opportunities that come with xml
binding, to far outweigh performance considerations.  

See the classes
   java.beans.XMLEncoder,
   java.beans.XMLDecoder