Maybe this helps: not talking about scalability to 1000 users or so but maybe a nice example of Alfresco in 'practice':
We have 6 small companies, each with their own hierarchy and connected to a central datacenter over IP-VPN and Lan. They are running in one instance of Alfresco. AD-authorisation tested and working but not necessary yet.
What do we use Alfresco for?
Simple filestorage over webdav/cifs en webclient.
Centalized mail distribution for all the companies (departments and personal) via imaging (canon/hp scanners) and Kofax Ascent for capturing imaging and distribution to Alfresco where some rules and scripts distribute the documents and meta-data from Kofax.
Next to this we distribute incoming faxtraffic for these companies in a comparable way for the end-user: faxnr coupled to company, faxservice drops files om Alfresco share and rules on share notify faxmanager (per company) to address fax-enduser.
All users have personal inboxes (spaces within Alfresco) in which rules report via notifications to the respective users.
This part, setup in practice as a proof of concept, preliminary to offering such a solution/setup to our customers works flawlessly for about 4 months now.
Depending of specific needs in usage cases you could consider setting up multiple companies within one instance of Alfresco, downside is more complicated user-management, or setup multiple instances of Alfresco, using authorization via ldap/ad etc.
Maybe this adresses part of your questions by example.
regards,
Peter