Really long post, but I started typing and had more to say than I realized.
I have used one on every single run for a 2.5 years now, and I love the data that I get from it, with a couple of caveats.
I don't train by power, either in planning or on a day to day basis. My training is based on a combination of time on feet or mileage, heart rate, and pace. I do use the power as metric to gauge fitness, and I also use it to reconcile that output with other metrics after running.
I really like the running stress score that you get from Stryd and find that to be considerably more informative than recovery time from Garmin.
I have never had issues on a treadmill so long as I am at lower incline. Stryd will not recognize incline on a treadmill, so the power needed to run at a higher incline won't be recognized. There is some convoluted way to use the Stryd profile on your watch to manually input the incline for segments of a run, but it's way too complicated for me to try to deal with it. The unfortunate thing for me is that most of my treadmill running is to simulate hills, so I am losing something by not getting a correct power reading. It bothers me, but I've just gotten over it.
The people at Stryd are helpful in that they will help you find whatever you are doing wrong so long as it's something other than their product. I had one race where I got bad readings (and therefore results that looked better than they were). I reached out to them for help, and they wanted me to go through a ton of providing data, manual testing, etc etc to help them help me. I found it to be very defensive and unhelpful as mentioned by OO. In the end, I didn't have the Stryd attached properly (offset laces on the Alphaflys), but it was a pain in the ass to get to that realization. No issues with either the Vaporflys or Alphaflys now that I know that.
My overall view of and experience with Stryd has been great. I won't run without mine on at this point, and I really like and use the analytics that I get from it. I am sure that if I really set my mind to training by power that it could be a good thing. I just haven't ever done that.