The gear sensor works great! There is no sign of it being set off by minute movements of the gear cable - even when bumping through deep potholes, it doesn't cut power to the motor in error. It's situated near the rear axle and secured using a ziptie - I may use a 3M sticky pad underneath, but the thin sponge might introduce additional movement, so I might not. I've very pleased indeed with this gear sensor! It's so nice not hearing grinding of the rear cassette when powered changing gears. When the gear sensor does cut power, it does so momentarily, after which the motor resumes power at the same rate as before - the slower onset of power as set in my firmware seems to be ignored, but this doesn't seem to be a problem. It all took rather a long time for me to install because of the slowness of the post... I could have simply walked to the bike shop to get inner and outer cables, but I refuse to leave my house! So it took rather a long time for everything to be delivered. People may have experienced problems with the gear sensor cutting power because they left too much play in the gear cable - getting very clean cuts and snug, solid, ferrule end connections is key, I think. I had to wait rather a long time before I got clean outer cable cuts because the heavy duty cable cutter took so long to arrive - long enough for me how to work out how to make (very untidy) cuts using two pliers: pinch the cable extremely hard in one spot with one set of piers, then bend the cable back and forth as fast and as hard as possible with another set of piers to fatigue and break the cable, then cut any remaining individual strands; this makes for very untidy cable cuts indeed, which are very hard to fit into ferrules (plus you end up with a lot of very hard cable steel strands all over the place - I found one sticking into my foot through my trainers). I think that clean straight cuts avoids gear sensor problems.
The gear sensor is well worth installing. Kits would be more complete with one included. I wasn't looking forward to the onerous task of recalibrating my gears again, but after a rather lengthy gear tuning session, all of my gears now work perfectly. I might use pre-stretched gear cables next time, to save time.