If ever you were asked to define a perfect plugin - this would be it! Just install it and it works as promised out of the box. No need to configure loads of settings, no resultant issues to deal with, etc.
This has overnight brought down my spam from 60-80 mails per day to ZERO. Yes, that's right - ZERO. For the first 2-3 days I was worried whether this was blocking all comments and not just spam, but that has since been proven not to be the case. I am getting genuine comments as usual, but all the machine generated spam is GONE!
All in all - the most perfect plugin I have ever come across!