...the blocklists are open to the public, and thus parties who may wish to circumvent PeerGuardian can actively check the list to see if their IP addresses have been blocked.http://en.wikipedia.org/wiki/PeerGuardian#Other_criticism
The blocklists are also managed by the public, but there is no fool-proof method on checking or reporting why an IP address or range are bad, nor on checking if the blocked IP addresses still remain bad. The list relies on the public to make submissions, and thus is vulnerable to attack itself...
You can basicly say the same about PeerBlock.
I don't use this tools anymore. A normal (desktop) firewall blocking malicious network activities and blocking applications to connect to the internet does the job.
A (desktop) firewall gives you a wrong feeling of security as well. Certain tests shows that most (desktop) firewalls are tricked out very easily.http://www.matousec.com/projects/proactive-security-challenge/results.php#products-ratings