The ECT button does exactly as said in the first post, as for the fuel consumption (I wont use figures as we use KM's / Litres not Miles / gallon) but the consumption all boils down to your driving style.
The ECT power on, gives you more power, if you drive more aggressively with this power, consumption will go up. If however, U drive the same style as when u drive with it off, consumption will go down.
E.g. If u travelling up a hill and gear in low revved 6th, it's going to take more petrol to maintain the speed than what it would if u were in high revved 5th.