"Zero false positives" is NOT TRUE!!
We use a reliable theme, Weaver II Pro.
We have a very busy site (http://ssnet.org) with lots of genuine comments. However, the spam that Akismet put in the spam folder got annoying and the promise of spam rejected before it was picked up in the database was tempting.
So we installed the WordPress Spam-free Plugin. We inactivated it after 6 hours with no comments, when I thought we should be getting comments. It seemed strange.
I wondered if there could be a conflict with Akismet, and the author said that it would run with Akismet but suggested that I should replace Akismet with spam-free.
I was reassured by good reviews and re-activated the plugin.
I tested the installation with a number of spam comments correctly identified by Akismet. Spam-free let through them through if they did not contain links in the body. Akismet didn't.
Spam-free correctly rejected comments with lots of links. It seemed that we needed both plugins.
The spam queue remained empty!! Great!!
After working fine for a number of comments, one of my lengthier comments got rejected because I was "posting too fast" and the comment was deleted and unrecoverable. Not sure this was spam-free's fault.
HOWEVER, my husband worked for nearly an hour on a thoughtful, detailed comment, but it was rejected by spam-free for lack of a password or "incorrect password." (Spam-free is supposed to work with "anonymous passwords," whatever that means.) This was definitely Spam-free's fault, and the comment was unrecoverable. The "back" link went back to an empty comment form.
How many other users were frustrated?! We do not know.
But if you value thoughtful comments on your blog, I would advise against installing Spam-free. Instead, use Akismet, disable track-backs and moderate your comments.
If you'd just rather not be bothered or just love plugins, by all means use Spam-free. You'll never know the comments you missed!