This plug-in worked great for me. I was able to copy & paste 50+ users in CSV format into a web form (it can also load a file), it then prompted me for which columns to use, and then optionally showed exactly what it was going to do and gave me one last opportunity to make edits by hand.
This sequence gave me real reassurance that I wasn't going to be creating a mess. I was able to see that the user IDs would be what I wanted, etc. You can also default to a certain role, so that saved having to tweak a bunch of new users and make them 'authors'.
Very nice, very easy to use, very clear. Love it.