I love to use notebook style IDEs to prototype, test and benchmark "smaller" things. And with smaller I don't mean things that aren't complex or difficult, but

I love to use notebook style IDEs to prototype, test and benchmark "smaller" things. And with smaller I don't mean things that aren't complex or difficult, but things that don't need thousands of lines of code or ellaborate software architecture. They are great to try new algorithms or evaluate their performance. If something is not quite right the first time, just change a line and evaluate that block again, without having to run another, potentially expensive block, again. You get to see the changes to your latest block immediately, without having to go through all the previous steps again. They are also great if you want to put together some graphs or statistic displays. I'm still hoping for something like Mathematica for javascript, including ways to print graphs, meshes, point clouds, etc. to you notebook.