Personally, I never found this to be an issue. I also don't see the imperative correlation between "has been read" and "has been dealt with". However, I know how often users ask for this feature. Therefore the option to manage unread marks per user or per database would be desirable.

But looking at how long it took to makes unread marks finally work half decent, I wonder if this is something to expect in the near future ...

For one of my customers I have added an extra column to the Inbox that shows if a message has been read by any of the named users. A named user is one that is mentioned in the ACL with at least editor rights. This to avoid that a message is marked when read by an administrator.

It took me a long time before I finally got it working, but it is ok now.

I found the idea to have this mark somewhat strange. I would prefer an alternative method for dealing with "dealt with" messages. Why not move them to another folder? This could be a folder called Inbox-read.

That's what I generally end up telling users to do as well.
Personally I would be annoyed if a message being read by someone else would show up as being read when I haven't read the message. I would consider this a bug, but I've heard this question from a number of secretaries/PA's during an Outlook to Notes migration.

Being able to manage unread marks per database would be ideal for shared mail files. I have clients who need mail files where the responses are handled by multiple people. Using a generic id does not allow tracking the users. Yes we customize the design as Rob mentioned, but would be much simpler to have the ability to maintain unread marks per database.

@3 I think Ninke you have to put yourself in the position of a PA & Manager, both accessing the same DB both wondering who has dwelt with what email. Moving something that has been dwelt with to a folder is the only option at the moment which is cool but if I am the manager I now have to look in two places to see the activity of my emails. Perhaps an alternative is to show that the email has been read by someone else by either an icon or making it italics so you have plain for read, bold for unread, italics for someone else has read.