From my understanding, all IPython Notebook viewer does is convert a notebook to html, and serve the html.

So I am comfortable investing in this format, with the notion that I can dump to html and be no worse off than I was making blog posts or writing up tutorials.

On the other hand, IPython is 5+ years old(?), and it has some pretty strong backing and momentum. I have a good deal of confidence that it will be around for a while.

Edit: If you miss the comment below, IPython is almost 12 years old!