I've been using Torque + bluetooth OBDII for years. True, you do "just learn" how to drive efficiently in a few weeks/months, just as you learn to gauge your speed without constantly checking your speedometer, but the sweet spot for every car is different. My v6 minivan actually turns out to do better if accelerate more briskly after a full stop than my V8 luxury sedan, due to the different weight/power/tranny ratios. Also, US fuel stations pump a different mix in the winter (~5 mos, here in New England) with a different sweet spot. In both cars, I get up to 30% higher mileage than the other drivers in my family, ranging from teens to geriatrics, because I bothered to learn.
Also, several cars I've owned with an "instantaneous MPG readout" aren't well designed for this use. They may use a small built-in (limited) OBD2 display at the bottom of the instrument cluster (it can be distracting to constantly move my eyes that far) or switch out of Instant MPG mode when they feel they have "more important" things to tell me (like radio channel changes), then don't return to MPG mode.
As a hobby, I use an old android phone, tucked in the distant corner of the dash, where it meets the windshield, as a HUD projecting up onto a but or aluminum foil on the inside of the windshield (where it would only block my view of my hood/bonnet, not traffic, in accordance with local laws) Torque has a "HUD mode" (reversed image) to support exactly such a use. It's fun and useful.
I dreamt of such a device in my youth when I took long car trips in cars that were not always in the best of shape. Many a time I had to nurse a car home, wishing I had realtime info on something (battery voltage, oil level, radiator temp, fuel level -- sometimes it was a dash gauge that failed)
So, while it's not something I'd buy, I applaud the notion. More efficient driving doesn't have to be obnoxious (but they do seem to push the idea of "beating others" or "personal best")