I downloaded this plugin version 1.0.2 and installed on my local development pc. WordPress 3.0.4 with twentyten - pretty basic.
When I activated this plugin it spewed out a header of
CSV User Import
All users appear to be have been imported successfully.
It seems the file was not uploaded correctly.
The plugin generated 3483 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
Please select the CSV file you want to import below.
The CSV file should be in the following format:
firstname, lastname, username, password (plain text), email address
Please make sure you back up your database before proceeding!
All this before my usual WordPress admin header and then my page was distorted with my column layout removed and all text in one vertical column, left column first. Uninstalling and re-installing caused no change. I couldn't even get as far as uploading a csv file. Shame as it's a great idea and would be so useful.