So Ben was lying all the way up to the end? because they get off the island.
And it looks like Locke was right about staying on the island because future jack is anxious to get back there.
I think that's why Ben wasn't lying. Being on the island did something to them and so leaving the island is going to kill everyone that was on it (either by having them go crazy or that you could live forever on the island). That's why Juliet couldn't leave and Ben wouldn't let anyone else go.