You can use it to block not only individual computers but also entire countries. Best of all, you can copy one of the many IP filter lists available on the Internet to give you a head start on protecting your computer from some of the most 'dangerous' known elements that other users have already discovered and recorded.
Keep in mind that although PeerBlock works well and is pretty secure, it does not offer 100% protection. The website itself will remind you that the only way to be completely safe is to avoid downloading anything copyrighted in the first place.
PeerBlock is a useful application for frequent users of P2P networks, since it will add an extra layer of security to your downloads.