I haven't used Emacs, but I've worked with people who have, and the one thing that made me think 'wow' was this. So many times have I yanked something, deleted some white space or some such, and then lost it.
Oh yes. The only thing I wish vi had was a key that let you traverse these so you don't have to remember exactly how many yanks ago you yanked something.
If (like me) you are a Vim user, there is a plugin to replicate this functionality called YankRing. Highly recommended! http://www.vim.org/scripts/script.php?script_id=1234