You want a spam filter to knock out all the spam but never stop a genuine comment. WP Captcha-Free has knocked out all the spam I was getting, and when I've tested it several times myself or via a friend, the comments were accepted no worries. So it seems to be working perfectly, but of course I can't know for sure if any genuine comments are being stopped.
It is quite invisible to the user, the only thing you notice is a slight delay in a comment being accepted.
I think it's brilliant.
Update: 1 year later, I'm still immensely happy with this plugin!!!