According to this post the accuracy of GPS (Global Positioning System) is likely to degrade over the next few years as the original satellites age. This will especially effect GPS in environments such as mountain valleys where fewer satellite signals are detectable.
If you are new to GPS you must ensure that it is set up correctly before use. After switching on you also need to give most units time to detect a number of satellites before relying on the position they give (my unit has at first positioned me a couple of kilometers away before correcting its mistake!). In my experience GPS units that mark your location on a moving map display (using Memory Map or similar) offer faster and far more intuitive navigation that is less prone to human error. However, I'd encourage you to develop good map and compass skills: they are less prone to technical failure, and learning the art of navigation (getting lost) can be entertaining in its own right.
For detailed information on how GPS works and its accuracy, click here.