Que serviço de filtragem de comentários-spam funciona?

Question

De uma resposta que dei a outra pergunta:

There are comment filtering services out there that can analyse comments in a manner similar to mail spam filters (all links to the client API page, organised from simplest API to most complex):

1. Steve Kemp (again) has an xml-rpc-based comment filter: it's how Debian filters comments, and the code is free software, meaning you can run your own comment filtering server if you like;
2. There's Akismet, which is from the WordPress universe;
3. There's Mollom, which has an impressive list of users. It's closed source; it might say "not sure" about comments, intended to suggest offering a captcha to check the user.

Para mim, estou feliz com a filtragem off-line da mão, mas sugeri o serviço de Kemp para alguém que teve uma experiência decepcionante com a Mollom, e gostaria de passar mais relatos de alguém que tenha tentado essas ou outras serviços.