Short answer: I don't use it. Eco mode has been on for about 30min total in 2.5 years.
The Leaf is far and away the most fun I've ever had driving a car. Being able to take almost any ICE car off the line, is just so much fun. I've done 45k miles in the 2.5 years I've owned it (new)...and I learned long ago (within the first few months), that Eco just brings this lively little sports car to a halt. Like driving through molasses. As others have mentioned, it does little to actually save energy, as you wind up having to simply press the pedal way harder, just to stay with traffic starting from a light/stop sign type thing. For me, it's pointless. I used it very briefly to try to eak out some miles when my wife and I took the Leaf round trip from San Diego to Portland, OR (fantastic trip btw)....and that was only when I was in cruise control on the freeway.
With all that said....my style of driving the Leaf has turned into me actively using the ePedal switch, a lot....on & off. I prefer having the ePedal off, B mode on for almost all driving. Makes the Leaf insanely responsive & quick, while giving you that bit of Regen braking from the B mode. It's a perfect combo for fun driving.
When I come up to traffic, or a light...I simply flick the ePedal switch on. Freeway driving is the same...99% B mode on/ePedal off. You still get that insane acceleration to make a lightning quick pass, or if you just want to get away from a gaggle of cars... The Leaf has the power to do it. In Eco mode...I'd think your foot would get exhausted having to smash the pedal all the way to the floor to achieve the Leafs true potential.
In a nutshell....I have fun driving this car. Been very happy with it. I'm also someone who fast charges almost exclusively, with some trickle charging at home from time to time. I've never worried about stopping at 80% or any of that. Battery bar still full, though I'm sure an OBD reading would give a reduced SOC....but I could care less. I'll keep this lovely Leaf another few years, then onto another fun EV adventure with another brand most likely (primarily to get away from Chademo)