I have used GPS on mobile phones for a while now to navigate to parts of fields where I have had trials located. I have always been surprised with the accuracy of phone GPS for this purpose and now there are some apps available to show just how accurate the GPS on your phone or tablet is.
Android
GPS Test is a great app with all the functionality you need for seeing exactly how your android device is using satellites and what positional accuracy it is giving. The number of downloads (well over a million) and Google Play rating (4.6 from +20000) is indication enough about the usefulness and ease of use of this app.