Akismet and TypePad Antispam both work in similar ways, in that they are external web services. All comments posted on this site, with the exception of those posted by people who sign in with OpenID/Facebook etc. and whose accounts I have trusted, are passed through both of these web services and each reports back as to whether they think the comment is spam.
Initially, I received basically the same results from both – i.e. both services would agree a legitimate comment is fine, and a spam comment is spam. More recently, Akismet would correctly identify spam but TypePad would bring up false negatives – i.e. it would pass spam messages as being legitimate. At the time I had the two weighted the same, using MT’s junk scoring system, so any comments where the two services disagreed would be allowed through as each service would cancel each other out. As Akismet seemed more reliable, I increased its weighting.
But recently the tables have turned. More spam comments have been coming back negative from Akismet, but TypePad has been correctly identifying them as spam. Unfortunately as I have Akismet weighted more highly, the comments still tend to get through.
Both systems have a feedback mechanism, so I can manually mark messages as spam and this is reported back to the web services, which should help train them to be more accurate over time. And it may be that Akismet is just going through a ‘blip’ -. Thankfully neither service has resulted in a ‘false positive’ yet – i.e. a legitimate comment that it thinks are spam. Still, although the services may disagree from time to time, it’s nice to have more than one opinion about a message – relying on one system alone would have probably let through more spam. If you have a blog, you should definitely consider using both.
(Incidentally, although TypePad Antispam was written by Six Apart, there is a WordPress plugin available too)