I have used this plug-in for several years and absolutely love it for its effectiveness and simplicity of use. Just installing it will immediately drop spam comments to virtually zero.
In fact, whenever I see spam comments on my site again or awaiting moderation, it was usually the result of an update to WordPress or NoSpamNX. Simply deactivating the plug-in and reactivating it is all it takes to stop future comments.
The technology behind the plug-in is brilliant. It creates a comment field that is only visible to spam bots. If something was entered in that field, that means the comment was generated by a bot, not a human, and instantly deleted.
Of course, it doesn't work when a human finds time to pepper a website with spam, but this plug-in is remarkable for eliminating the bulk of spam comments. I highly recommend it.