To use it, click the up and down arrows beside search results to re-rank them. Next time you search for the same thing, your results will remain in that order. To comment on a search result, click the comment icon.
Google is quick to say that any changes you make to the order of your results does not impact impact anyone else. Comments, however, can be seen by everyone, which could be a haven for spammers.
I'm still a bit confused why people would use this new feature -- to me it seems like a huge waste of screen real-estate considering the vast majority of users will never touch it. Even fewer will likely rank web results and do the same query again in the future to see any benefit from the service.
What do you think of the new SearchWiki functionality?