I have been on a couple of rides with the range extender battery.
I charged the new battery and the existing battery fully before installation.
Installing the battery on the bike is fairly simple though at 1.6kg it's more of a brick than a bidon. It is secured in the holder cage by two lego style 'plugs' on the bottom and a bottle holder style cage with a rubber stretch band to stop it rattling about.It is quite secure and does not rattle. There is a electrical plug that connects to the charging point on the bike. The range extender has its own charging point, this can be used to charge it off the bike or you can use it to charge both batteries on the bike.
My initial thoughts are, forgive the pun, it is a little ungainly. Nobody will be under any illusion, it's an ebike, you can see the battery.
Before I added the extender, I had the engine set at 80% on the green setting, 90% on the amber setting and 100% on red.
I whacked up all engine settings to 100%.
The ebikemotion app told me I was 200% charged.
If you have a Gain, you will be familiar with the IWOC button on the crossbar. There is one of these on top of the battery. You need to turn both on.
Ride one, 66 miles, 5600ft https://ridewithgps.com/trips/38499937
The bike feels heavier, the weight makes a difference to the handling. I've done many miles on a heavy ebike - it's not that bad, just different weight distribution, I'll get used to it. Usually I try to spare the battery for the first few miles. This time I was a little more free, averaging 8.5mph up the first 3.8miles uphill as opposed to 6 mph previously. The range extender was empty at 29 miles 3200ft into the wind. I had 100% power left and just 37 miles 2700ft and a following wind. It was good to be able to power up the hill out of Hathersage in amber with the occasional dip into red.
I got home with around 40% of battery to spare.
Ride 2, 66 miles 6200ft https://ridewithgps.com/trips/3854678
I had charged both batteries in situ and did not check before the ride how much charge I had. The ebikemotion app is a bit unpredictable and doesn't always connect. I had a stop at Baslow,7.8miles, and checked there to find I had a mere 150% left. I changed the engine setting on eco (green) to 85%. I'm not sure when the extender battery ran out but it was quite a strenuous ride and I needed the amber button to get me home the last 11 miles. I had 37% power remaining at the end of the ride.
I put the bike back on charge, batteries in situ, and it took a good 5 hours to charge back up to 200%. I feel I have still to get the measure of the new setup. It is worth remembering the extra battery is giving more range, not more power. You won't get any more than 40 Nm torque out of this engine. Over about 10% you have to put in effort. Whereas with an 80Nm engine you really can breeze up the hills.
It has extended my range but the battery anxiety has not yet evaporated. I can be ambitious in my choice of rides and can always press the 'get me home' button. I'm not sure about having to carry a great lump of spent battery for the second half of the journey.
Is it worth it? If you have to get out there on long rides in demanding landscape probably.
About the rider: age 73, weight 142lbs, condition, not absolutely perfect.
Hope this helps.