For what it is worth, I bought a PowerPod for one of my outdoor bikes. It doesn't measure strain in the usual sense - it measures acceleration, climbing/descent, and it has a wind sensor in the front of it, so if you are riding into a headwind it will account for that. When I first got the thing, I needed to do a calibration ride.
I went with this thing mainly because none of the other options would really work very well for me on this bike, and the price was pretty good.