I use the Planet3 device and it was super easy to fit. The guy who makes them posted a step by step video complete with comical commentary on YouTube that walks you through all the steps, you will need a lockring tool to remove the old magnet and a torque wrench to set the correct amount of torque (I borrowed one).
I can safely say that with the device installed, it's way more capable. I've only done one quick ride with it installed but will get out again this week and see how it affects battery life. Generally, I ride in Eco 60% of the time, Sport 30% and Turbo 10% (for the hills). I also find that I don't need assist a lot of the time, so I sometimes turn it off for half the ride). On my last ride, I did 100km + 1000m of climbing without the derestrictor and by the time I was home, I had run the battery totally flat. It's also super windy this time of year in the south of England so that probably saps some of the energy. I hope to have an update in a week or so. I've attached 2 photos showing the device, it's super simple and you barely even notice that it's on there, great design.