Bit of a novelty if you ask me. My mum had one in her 2007 Fiat Panda and all it did was slow the gearchanges down (it was a semi auto) and soften the throttle response so you cant accelerate hard. Made little difference. You could save more fuel using your own techniques
It's the placibo effect (excuse spelling) if you think you are going to save fuel by pressing an button with eco wrote on it, you probably will. My car has a similar system, sport and non sport. In sport mode the steering becomes heavy for better feedback, the throttle response becomes sharper and engine torque increases by around 20%
Strangley though, I use less fuel in sport mode, its more about driving style than electronic gimmicks
Incorrect to say it's a "placebo" effect. I own a 2012 Accent and have Active Eco on 100% of the time. What it does is adjust shift points and throttle response when you accelerate to save gas. Its gains aren't huge, but certainly worth it if you bought the car for fuel savings.
I am a little curious about saving "at least another 25%" with a manual transmission. Most of the literature I have seen suggests about 10 - 15% penalty with automatics. And some of the later CVT automatics, at least in EPA testing, post better MPG than manual transmissions. Even if the automatics have worse fuel economy than manual gearboxes, many of us are willing to pay the "convenience price" of not having to deal with the manual gear changes and clutch in heavy traffic. Before you ask, yes, I can drive a standard transmission.
Well thats juat typical lazy Americans for you, moving an extra hand and foot is far too much effort?!
I find manuals far easier to drive, even in traffic, you only have to be in 1st gear and move your clutch up and down. In an auto, its accelerate brak, accelerate brake.
As for the fuel savings, if you drive like a zombie on auto pilot, then yes, you might only save 10%, but having a manual allows you to drive the car, not the car drive you. When I pull out at a junction, from 0-60 takes me 1 gearchange, not 5 like an auto would choose. I pull off in 2nd gear, then change to 5th at 30 MPH and then accelerate to 60. And obviously, its easier and safer to go into neutral in a manual on hills, and use gravity instead of petrol!