I think it depends on what sort of person you are as to how much EK will benefit you. If you are mindful about when you run things, you will save a bit. If you are fanatical, you could save a lot. 

I find that Winter is the best time of year for saving money with EK as heating will generally use a lot of power so running heating during the free hour will increase your free usage proportion. Over Summer, the savings were much less. As someone touched on, it could encourage people to be wasteful, "must use the power because it is free", so there could be some downside to it.

It is reasonably easy to remember to run the dishwasher / dryer etc during the free hour though. It also gives you the chance to get the house warm during that time. The % also depends on how much power you normally use throughout the day. I don't use much (low user), therefore, the hour of free power can use a higher proportion of the days power as the rest of the time the usage is low.

My overall average is 15.7%, and that's just from being quite mindful of when to run things to maximise the benefit. 

In my area, the stay ahead 200 plan seems to be the best overall deal. Even without the free hour, the unit / day charge was the best. 

Having said all that, there was a day where I went all out to see how much I could use vs other times (was heating / cooking / dryer - all useful, not wasteful). Here are the results below (47.8%).