I came back from my ride a couple days ago sold on the Microtune feature both for utility and battery savings. Overall, I rode 30 miles and used 13% battery. Well, that's actually not accurate since I rode 12 continuous miles out to our destination with the motor entirely off just to see how it compared to the bike I've used on that route for several years. The conclusion on that was that there was little difference, especially since our group tends to ride at only 12-14mph. And still, I did ride with assist for 18 miles and used only 13% of the battery.
Not counting the initial 3 miles that I rode in Eco, the 15 mile return trip was done with Microtune on flat trails (11 miles paved) and into a bit of a breeze. I found Microtune very easy to use, and could bump the assist up and down easily to match the conditions, as opposed to having it in one of the standard modes that might be more than I needed at times.
Note 1: The TCU display changes to show the percentage when using Microtune. However, the Mode field on the Garmin doesn't know about this and what it does is display the normal 1, 3, or 5 depending on the range of the assistance, For instance, at 10%-30% it displays "1" ("Eco"), then at 40%-60% it displays "3" (Sport), etc. Maybe this can be fixed in the future.
Note 2: I can confirm that, when set to 10%, the motor does switch off when the pedaling effort is light. I configured one of my TCU screens to show the bike's motor and electric power. When pedaling easy at the 10% Microtune level, the bike is quiet and the display shows 0 for both power readings. Pedal a little harder and the motor comes in with about 12-14W added. So the bike is not going to bother assisting at some miniscule amount that wouldn't do much for you anyway!