My experience differs, I'm afraid! We tried postgrey, but didn't get on with it (I forget the specifics now), so we tried sqlgrey (http://sqlgrey.sourceforge.net/) and it's extremely effective. Grey-listing has to be the most effective anti-spam tool there is at the moment.

Fully configurable, sqlgrey also has a nightly update cron job, and supports whitelists.

We run Postfix, BTW.

I use Postgrey and find it really easy to use.

Simply set up a few whitelists, like yahoo.com, that you trust and set how long you'd like mail servers to retry the mail's delivery and you'll drop spam and hardly see any false positives if any.