Look how deceptively easy the Guice User Manual makes adding Guice to a Struts 2 app appear. Look, it says, just add one jar and all your dependency injection needs will be meet. Of course what it fails to tell you is that in addition to adding the plugin JAR, you also need the base jar, the servlet jar and if you want it to work you need to download a fixed plugin jar. At one point it was moaning about commons-logging as well!
With all that done though, it does indeed work as advertised.