Mollom does not only protect you against comment spam, you're also able to protect the user registration form, contact form and other user generated content. If you're looking for a decent antispam solution, you should certainly give it a try.
I've been using Mollom alpha for a few months on this blog (lot's of spam here), Tik vzw and a few other websites and I was very pleased with the results. More than 20.000 spams got blocked on this site without any false positives. Badly enough, the best part of my stats got lost during the alpha testing, but I still have a nice graph for tik.be:
That's bad luck, just after the launch the LCL Diegem datacenter went partially offline due to power problems. (everything is back online.)