It definately works on my client websites. However I still get a few addresses that get past the security. The reason is because of the services available.
If you login and go to Block Country panel you can see at the top 3 IP to Country Services. I would choose geoplugin.net as it seems to catch the most IP addresses.
Unfortunately, you can't block all addresses, because new ones are always made. This plugin has reduced attacks on one of my clients sites from 100 per day, to about 3 a day, thats much better.
Also I think you should download and use Wordfence Security. The two plugins together works really well. With Wordfence, if someone gets past my Admin Block Country plugin, you can use it to further block intruders.