Mollom has done a great job for my site. It blocks approximately 131653 spammers in the past 227 days. Apart from that, I have also manually blacklisted some bad robots, and spamming IPs through .htaccess, which may not reveal through mollom statistics. I always verify through project honey pot or whois.org before banning anything. I am currently on a shared hosting environment, so every single byte counts.
