"Recent findings by researchers from the University of California, Riverside, show that 15% of the IPs people connect to on the Gnutella P2P network are blocked by blocklist applications such as PeerGuardian. Statistics like this do not prove anything about the effectiveness of these lists, however, according to an insider who worked for several anti-piracy organizations, blocklists significantly decrease the risk of getting caught by the MPAA or RIAA."
TorrentFreak, which is becoming a popular source for P2P news (least for me), has recently posted an article that investigates the use of applications that block 'bad' IPs. The 90 day study from University of Cali. came to the conclusion that "a user who is not using blocklist software is practically guaranteed to be monitored." The network they used, however, was Gnutella--commonly used by filesharing clients such as Limewire. Go figure.
Of course nothing is foolproof and according to TorrentFreak, who managed to catch an expert in the field, guessed that 75-80% of the IPs used by anti-piracy companies (that he worked for) were on the blocklists. Something is better than nothing I suppose.