I am worried about allowing more than one user of my Wordpress installation use Wordpress at the same time. What if one user updates a page or deletes a page while the other user is editing it? How robust is Wordpress under this multi-user case? I am talking about users who are logged into Wordpress as editors.
As far as writing/editing pages and posts WordPress is pretty darn good at handling multiple users and conflicts. There are clear warnings and error messages to the user that someone else is actively editing the post/page. And it will prevent you form saving changes if another user opened the post/page before you did.
The one drawback, as with any multi-user system, is those times when a user opens a document for editing then runs out to lunch or home for the day. No easy way to unlock the file that I'm aware of.
To see it in action simply attempt to edit a post/page from two different user accounts and browsers.