I have Daihatsu Boon 2012 with Eco Idle feature, the only thing it do is that it switches off the engine when the car is stationary. Other than that the car is identical with the regular model. So the amount of fuel saving depends on how much time you spend idling, if you mostly drive on highways or signal free corridors, the difference would be minimal. but if you stop at a lot of signals, the saving would be noticeable.
One thing however, that's particular about Daihatsu/Toyota's eco idle is that it doesn't function for the first few minutes of the trip, when the engine is not warm enough. So it usually doesn't work the first two to three intersections after I leave from home in the morning. I hope it helps.