Seems very handy - it'll get better as more people get to know about it. Also, for marketing purposes it's useful as I understand Sidewiki content will be indexed by the search engine.
in reference to: Google Sidewiki (view on Google Sidewiki)
Strangely, Sidewiki doesn't work with Google's own browser, Chrome. I expect they're all too busy playing with Google Wave, or Gwave as some people are referring to it now!