IPython Notebook is an amazing tool. I used it a lot while presenting examples during my GSoC project. It was easy to clearly present the features I was adding to the library as the project progressed. I also used it regularly as an REPL, though I would prefer lot more keyboard shortcuts be made available.
Note that we also intend to at least expose Emacs/vim keybindings for the text area as soon as possible. It's not hard as codemirror has most of the bits we need, we just haven't had the time for it.
This smells like lots of hours wasted in the near future; I always wanted to get my feet wet with statistics, probabilities, etc. and it seems there's some nice content here.
Some examples: http://rhoforsympy.wordpress.com/2012/07/16/week-8-trace-imp...