I haven't tried Struts 2 yet. Preliminary test builds are all that's availalbe now, and I'm waiting for the production release. As I read the documentation I get the impression that the big difference is in the way that the user interface is built. The custom tags are completely different because Struts 2 is based on WebWorks 2, which is another framework that is merging with Struts.
This link has a lot of good information about Struts 2.