I bought one of these for $35 a while back and ended up returning it. I just couldn't get it to work reliably. It would work once, and then it died. It especially didn't seem to like being in the presence of a Wahoo cadence meter, which just killed it dead. Often it would not start up by itself, and I'd have to remove it from my bike to get it to respond.
In the end it was more effort that it was worth. My Wahoo speed/cadence sensors just work, without issue. This did not, and the best that the company could suggest was to use it with some Cateye software instead of Cyclemeter/Strava/Wahoo Fitness apps, which is a shame because the Cateye software sucks a**.
Sometimes cheap isn't good value.