I am trying to get the user's speed from their Android device, but which is the most reliable way to do it?
There is the location.getSpeed() function that uses GPS; is this a reliable way to obtain the speed? Should I instead calculate speed manually using GPS coordinates obtained? Or is there another way that I'm missing to accomplish this?
It seems that the getSpeed() method is not always reliable, especially at low speed and when gps coverage is not optimal. You can have a look at this question and this one which are both about alternatives for getSpeed(). The android developper page however says that you'll get better performance by using the Google Location API.
So it appears that the choice is depending on the usage of your app: if you target slow displacement in area with poor gps coverage (walking in the wood), use your own implementation. Fast in area with good GPS coverage, use the Google Location API.