One of my projects use it (http://eatmycharts.com).. I'd say it removed the registration barrier, but that's about it. People will come back for the service.

I can't track which users publish to their mini feed (is there a way to tell? you ask the user's permission and there's no javascript callback to know if the user approved on publishing). Most of my users that are in my facebook network prefer not to publish to their mini feed.

I myself don't like to use my Facebook account on other websites (unless I really want to use my identity on purpose). That's why on my website, I automatically assign a nick to the user (which they can change), so their full name won't appear on public.

Overall, I don't think FC helped a lot in my case.