NoSpamNX used to catch nearly 99% of all the spam certain small sites received in 2010-2012. It still catches some spam without requiring almost any setup, but in a short test from this spring, the rate of spam recognition was down to 32%.
I've recently labeled over 100 spam comments that NoSpamNX has let through, so it seems that this plugin is not up-to-date as the scripts that spammers use. Stop Spammers is on my list of plugins to try out to fill the same need.
This plugin still catches some spam, doesn't need any setup and doesn't bother users, so it has its uses. If only the spam recognition rate would be as great as it used to be, this would be back among my favorite plugins, easily worth 5 stars.
Right now I recommend looking around for a more up-to-date solution.