In competitive play, OHKO attacks are frowned upon, and it also never works because human players will just switch out. But still, 30 accuracy isn't that bad, and if you get lucky, the other team will have lost a member. That is why OHKO attacks aren't used by experienced, real players - because they rely on luck, and are generally cheap.
However, ingame I think they are fine. They also work wonders in places like the Battle Frontier because the AI seldom switches. In fact, I'm currently training an Articuno solely for that purpose. Now if only he had a decent recovery move....