It works fine for me, much more stable than 0.4.6 under MacOS X 10.0.3.
I got it working by downloading the old version of macphex (from www.versiontracker.com) and replacing the phex.jar file in it with the new one from 0.5. I also copied in all the new .jar and .class files from the 0.5.0 release.
If you want to make it look good also, you can edit the properties file to change the application name.
All this is best done from the command line or by control-clicking on the macphex application and relvealing the contents.
HTH