I was able to find and book an extremely cheap fare ($271) from LGA to AUS two days before the outbound, when other fare quotes from the airlines or my travel agent were all north of $1,000. So I would contend that Orbitz can be of real value.

At other times, however, I have seen better fares/less restrictive rules from other sources, particularly on international flights.

Net, net? It's another source worth checking, but it shouldn't be relied upon solely, any more than the airlines' web sites, online travel sites, or one's travel agent.