If nofollow works it does not prevent comment spam but works against sites linked to in comment spam from appearing in search results. For what it’s worth spammy search results seemed to me a growing problem perhaps a year ago. I haven’t reached a spammy site from a Google search result in a long time. If my experience of less spammy search results lately is not anomalous, has
nofollow helped achieve this? Only Google and near peers are in a position to know for themselves, but I certainly wouldn’t write
nofollow off as a failure, complete or otherwise.
Google should publish its findings regarding whether
nofollow has improved search results. If the answer is yes, web software creators and web publishers wouldn’t make the mistake of turning it off for untrusted links. If the answer is no it should be deprecated.
Addendum 20060601: The next time Google or similar ask publishers to do something the results of which can only be evaluated by the asker a committment to publish an evaluation should accompany the ask.
I should have said Google web search above. As Chris Masse points out in a comment below, blog search still stinks and so far Google and Yahoo! blog search do not improve the state of the art, contrary to my expectations.