Hi Phillip,
I used it on a few smaller sites and its worked without problems so far (about 500 visitors a day, 20-40 posts). The external link analysis is pretty cool and Ive been able to do away with captcha completely. Its also a fun tool to see what you can try and get past the spam blocker.
Its also instant and not like akismet where a response is given in 30+ seconds after something is submitted. Ive started to look at the API but haven’t implemented it yet, are you using it on any public sites?