right now we have that installation for three weeks, and I have migrated 10.000 users from different platforms to our central Moodle MNET authentication provider.
All the other Moodle instances have got only MNET login. There aren't any local users anymore. I am quite enthousiast about this solution. Because I can now login to all the platforms with one login.
Bulk enrolments is possible with a feature called "Remote enrolments". On the moodle installation, where all the user accounts are created, you can enrol the users to courses of the other Moodle platforms, even if they did not login to those platforms yet.
A workflow is described at:
I have also a modified version of the standard theme. That theme allows you to simplify the login on remote hosts. It hides the local login form and generates a login link with automatic redirect to the page where you came from. I just tried to upload it here, but it is too big. You can get it from here: http://ubuntuone.com/1coVBiEBR2bvC1gxz32Sqh
(it might not be there forever...)
Using the theme might lock you out of your site! (loginform is only hidden, so you can make it visible with firebug, change display:none to display: block).
You also have to define your site admins in the config file, when you use that theme, because remote users can't be defined as admins in the web interface. $CFG->siteadmins = '234,3,21,24';
The numbers are the user ids of the remote users who should be admin. We have a complete workflow how to migrate multiple Mooodle platforms to SSO authentication described in my first post. But the documents are in german.