I have now completed six days commuting and have done 234 miles since putting the 42 teeth chain ring with the Shimano Alivio HG400 MTB 12-36t: 12-14-16-18-21-24-28-32-36 cassette on. My first ride to work using this gearing was on the 28th of June and it is working really well.
The gearing is low enough to climb all the hills I have encountered on my cross country route in eco, and the steepest / most technical hills require bottom gear so there is no lower gear if I were to need one!. However I only need bottom gear on the two steepest / most technical sections.
On the road I am covering 10 miles with two loaded panniers at an average speed over 19 miles an hour and on a good run approaching 20 miles an hour, 95% of the time in eco, and the motor switched off on down hills and standard used only occasionally on the steepest up hill sections.
I am using just under 30% of my battery on the 10 mile hilly B road and as little as 25% on the 12 mile cross country route.
The Yamaha system would allow me to fit a double chain ring set up to give an even wider range of gears as Chris_n describes above and I might be tempted to do that at some point in the future.
Although my average speed is just where I want it the range of speeds I do on my road commute with this new gearing is wider. A lot of the hills I climb are in eco and my speed drops to around 11/12 miles an hour. I sometimes pop onto the same hill half way down that I climb at 12 miles an hour to connect between tracks on the way to work on and coming down that same hill I can top out at 36 miles an hour with the motor off. The high top gear allows me to keep adding pedal power with the motor off should I decide to do so.
There are also a few sections where a slight downhill gradient allows me to pop into my highest 12 tooth top gear in eco and maintain 24/26 miles an hour for the odd quarter of a mile+.
As soon as I notice I am needing to add more and more effort myself to maintain the speed in this gear I change down to the 14 tooth gear 8, to maintain 19 to 22 miles an hour so that this gear gets the most wear in terms of dealing with the highest combined power output of me and the motor and I hope is more capable of coping with it having 14 teeth rather than 11 although exactly how long it will last before slipping time and miles will tell.