It would be cool if more games implemented it. You just don't know when something cool will happen that you want to see again or show to other people. It could just save a flash file after every game like Halo 3 and delete it later. Speaking of which: replay features as deep as Halo 3's or Dissidia's would really help the people who like making machinima or turning matches into cutscenes.
Edit: I can't believe I forgot to say this (I've already been thinking about it way before today). A dynamic camera replay feature would be awesome for games like Devil May Cry or Ninja Gaiden. People already spend the time making tons of DMC combo videos anyway (and there's at least one guy who does for Ninja Gaiden). A dynamic camera replay function would be great for those.
