Comments should be very rare and valuable, almost always expressing the "why" and never the "how" (the exception being when the how is complex and not easily discernible from the code).
Every comment is a hint that you may need to refactor to make the code's intent clearer. Every comment risks becoming out of date as soon as it's written.
We have almost no comments aside from XML comments in our xUnit.net project, but some people seem to find the code clear and easy to read. :)
