I pay close attention to mine, and it feels reasonably accurate. I say "feels" because I don't actually reconcile my traffic count to theirs, but when I downloaded a couple gig yesterday, the usage jumped up within the hour.
I have noticed it's not terribly accurate. For the last few months I've gone over my cap because the Orcon meter has said I've still got xx GB remaining. Cacti (monitoring my modem) has always been more accurate, indicating I'd downloaded more than Orcon was telling me, but I'd been assuming Orcon should know better. I no longer trust Orcon, relying on Cacti instead. I realise Orcon's meter only reports up to the previous day, but I don't think it's accurate.