Having just purchased an OIII filter, I can say that it really helps with most of the PN I have been observing. A couple are barely visible without the filter, but very easy and clear with it. It isn't a case of all PNs are vastly improved, but I would safely say most are much better. It also helps me cut down on light polution when observing PN.
I would recommend it for someone interested in PN and it is good value for what it does (mine was $59 from Andrews).