I have used the free Mollom plan to protect the forms on this site with CAPTCHA or text analysis. For the past few months, it has been working very poorly to be honest. Comment posts that were very obviously spam were getting though consistently and I have been removing 20-30 spam posts en masse about once a week.
I submitted a ticket when I first noticed this happening with Mollom support and got a quick reply, but after my initial replies the request was waiting a follow-up from them for quite some time (and twice, at that).
Today I got news that it was a service-wide issue and it has been resolved. Hopefully you will notice a significant reduction in spam posts! Honestly I am not very happy with Mollom at the moment and may look to other alternatives if this doesn't pan out, but then again it's a free service and it worked wonderfully in the past so I can't complain. You get what you pay for.
The issue was more that I was looking at taking 5-10 clients to their basic paid plan, but after my experience with their customer service I am hesitant to do so. As always, I guess time will tell.