False positives are often non-zero, but generally very low. For the past eleven weeks, consistently under 1%. I suspect that this is due to the “easy on, easy off” removal policy-- If anyone trying to send you mail receives a bounce message back from you referring to the PSBL website, it's very easy for them to have their sending IP address removed from the list.
Even though PSBL catches a lower amount of spam (on its own) than some other more well-known blocklists, it manages to catch some spam that those other lists do not. To determine this, I took the last thirty days worth of results, and looked for intersection and overlap between PSBL and other blocklists.