I tend not to trust comments. I much prefer good unit tests, assertions, well factored code, preconditions, postconditions, etc. "Executable comments" have the huge advantage that they cannot get out of sync with the code.

One of the many things that intrigue me about perl6 is that things like .wrap, POST, PRE, typing, etc. make this sort of documentation-in-code far easier.

Favourite comment of all time, which once lived in the guts of a large system I worked on for a few years, was :

;;; I don't understand why this works?
[download]

Since the customers had access to the source this was removed at the request of some management when it was noticed. The code it sat above wasn't refactored tho' :-)

You might also find Documenting Methods/Subs of interest.