I'm no hypermiler, so this might only apply to "normal" driving. I decided to try ECO mode after 18 months of driving in normal mode. After 2 tanks of gas (I planned to go 3 or 4 for this test, but I had to replace my tires yesterday so that screwed me up), I averaged 39.1 indicated, which is about 1-1.5 mpg better than previous tanks. The weather cooled off as well, so that might have helped too. For the test, I tried to keep to the same driving style and profile (mostly highway doing 75-80 and back roads doing 45-60).

Bottom line it looks like ECO mode maybe increases mpg by 1. The first day or two I did notice the weaker throttle response, and the A/C wasn't as cool, but after the first week I have to say I don't miss "normal" mode anymore. And it feels like plenty of passing power is still there when you need it. So while it's not a lot better, it's something, and I'll probably stay with it.

The only thing that still bothers me about ECO is there's even more throttle lag at launch - I almost killed myself trying to make a left out of a parking lot onto a busy road with limited visibility. It literally delayed about 2 seconds before the car moved, and by then the gap had closed and I had to hit the brakes before I encroached any further. My teen son laughed from the passenger seat and said I need to get a real car (he drives a Civic...)

 

I tried Eco mode once or twice when I first got the HS250h but found the throttle lag too annoying in city driving. OTOH, the Power mode proved very useful in hilly terrain when I drove from Winnipeg to Denver this summer. With cruise control engaged, the throttle response on inclines was much quicker in the Power mode than in Normal mode.