Just to let you all know, the newest version of the plugin is available for download from http://happynuclear.com/sandbox/amu/add-multiple-users-for-wordpress.php
This is a beta version of 2.0.0, which I'm hoping to refine over the next week or so for a general release. There are bound to be some bugs I haven't picked up, so please only run it on your test sites if possible.
If anyone feels like testing it out and giving me some feedback, please do so, I'd be greatly appreciative!
I'm hoping this version comes close to fixing some of the shortcomings of previous versions, but I'm trying to not make it too complex to use. Not sure if I've got it right, so I'd be happy to hear your thoughts.