Akismet is the "official" WordPress response to the soul-sucking rampages of blog comment spam. It promises to make spam magically vanish by harnessing the Hive Mind to banish spam en masse. But it doesn't work. I've been getting a fair amount of spam approved by Akismet as ham, when they are obviously spam. Not sure what's going on there, but I'd guess that since anyone can flag comments as spam/ham, that the spammers are getting in the game themselves. Total guess though.
A couple of weeks ago, I turned off Spam Karma 2 to see how Akismet performed now that the system has had a few months to "warm up". The result wasn't exactly impressive. False negatives, false positives, and excessive moderation.
I could live with a few false negatives - the occasional spam slipping through the cracks and appearing on my blog isn't the end of the world. But I've also had a couple of false positives. Valid comments banished by Akismet. I can manually resurrect them, but what If I don't check regularly? It'd be really easy for false positives to get lost in the sea of spam (ick).
Also, Akismet routinely pushes comments into moderation purgatory. Someone attempts to post a valid comment, to be rewarded with an "I don't trust you. Please wait for your comment to be blessed by the High Priestesses of Blog before being deemed worthy of being displayed here." OK, it's not exactly as rude as that, but the sentiment is the same, and not exactly conducive to conversation.
So, I'm going back to Spam Karma 2. It rocks hard, and is intelligent enough to block spam and approve ham without intervention. It even has an Akismet plugin for SK2 to let me harness the Hive as a last resort. But even that limited role of Akismet has proven to be the only weak link in SK2's otherwise impervious armour, so I've downgraded Akismet's influence from "normal" to "moderate."
Can't say enough good things about SK2. Since I first started using it, back when the world was young and Grandfather Bear roamed the forest, SK2 has nuked over 8700 spam attempts. About 100 attempts per day, and for 99.99% of them, I don't even get notified. And so far there have been zero false positives (it keeps the comments and I periodically eyeball it to make sure).