Yesterday, Akismet caught its 10,000th spam since its installation less than four months ago. It has an extremely low rate of false positives, and a very good rate of real positives. It catches well over 99% of the spam that we would otherwise receive. As for false positives, I’d say it erroneously flags 2-4 comments a week on average out of the 500+ comments that we receive any given week. (Those comments get held until we release them, typically later that day).
We’ve been blogging for about two and a half years, and have seen a fair number of spam blockers. Akismet is the best available, hands down. If you’re running a WordPress blog, I can’t recommend it enough. And if you’re not on WordPress, it’s a pretty good reason to consider switching.