I've been using WP Spamshield for about 3-4 months now, after one of my sites started getting hammered with about 1700 spam comments per day for 2 weeks straight.

It works so well by itself, I was able to get rid of Akismet on a couple of smaller sites. Before this, I was using a combination of Akismet and Conditional Captcha, but that massive attack overwhelmed both plugins, which sent me searching for new options.

I may try Zero Spam on one or two of my sites, just so I can compare it and Spamshield in action.