I'm also curious to see if anyone has actually used the Friends and Foes feature. I think it's one of the new things that came with the new forum set-up. It looks like the advantage of putting someone on your Foe list is that you don't have to deal with them or look at any of their postings.
This is the description that's in the FAQ section of the forum.
Friends and Foes
What are my Friends and Foes lists?
You can use these lists to organise other members of the board. Members added to your friends list will be listed within your User Control Panel for quick access to see their online status and to send them private messages. Subject to template support, posts from these users may also be highlighted. If you add a user to your foes list, any posts they make will be hidden by default.
How can I add / remove users to my Friends or Foes list?
You can add users to your list in two ways. Within each user’s profile, there is a link to add them to either your Friend or Foe list. Alternatively, from your User Control Panel, you can directly add users by entering their member name. You may also remove users from your list using the same page.
I like reading everyone's postings when I have the time. I really doubt that I'd list anyone as a Foe unless they really tick me off!
I use them quite a bit. In the beginning right after phpBB 3.x was installed I added all the moderators to my friends list. This way I could just click and send out a mass PM to all the Mods in one sitting. I don't use the Foe list.