Everyone who bangs on about yield is totally ignoring the fact that we do not have 'yields' figures, and CAA stats/load factors are the only way to gauge the success of a route or of an airport; it's either that or nothing.
JonEMA is right, however I believe you can more accuractely judge the success of a low-cost flight through load factors than you could of a legacy carrier. I think it was Stelios that said, as a rule of thumb, they need a 70% load to break even. That was a while ago though and things may have changed.