Although I'm a big fan of planner-mode and use it daily, I reccently saw the Google Tech Talk about org-mode and really like some of the things it does.
One of these is the ability to have a hierarchy in tasks so that you can have one main task, and a bunch of sub-tasks that all need to be done to finish the main task. You can sort of do this in planner-mode by having different files, but not really.
Linux Journal article about org-mode
A big org-mode tutorial
An org-mode screencast
Outlining your notes with Org by Sacha Chua