Such as new posts they start, replies, or both? Status updates and other site activity. Is there an addon that does this? Seems to be a powerful feature lacking. What good is it to follow somebody if it basically just adds them to your buddy list and nothing else. Would you be interested in this feature?
there should be more ACP-options for admin to define when a user should receive e-mails.
For my small community, I would need to have my users notified more often in order for them to regularly visit my forum. It is hard to build a community from scratch if users are not notified via e-mail when e.g. another user is following you, etc.