I also found this plugin to work as described. Using Joomla 1.5.7, athough I could not make it display the 'Message to other users' who should not see the text. see below:

{accesstext mode="user" user="Mark:Lena"}
Message to other users.||Message user Mark and Lena.
{/accesstext}

This link also helps with initial set up as there was no example to begin with that I could find:
http://www.greatjoomla.com/tutorials/core-design-access-text-plugin/how-to-setup-core-design-access-text-plugin.html

It also worked around {mp4} tags as we are using it to restrict access to certain movies.
Thank you.