Relying on GPS devices can erode our ability to develop mental maps.
While GPS technology can help us in a pinch, relying primarily on a system that does not engage our navigation skills will weaken our ability to perform these functions. While this intuitively makes sense, that the 'mental muscles' would atrophy when not used, it is a reminder that an overuse of geospatial technologies can be intellectually counterproductive.
A distinction should be made between outdoor GPS usage (where the user receives data and makes navigational decisions) and vehicular GPS usage (where the computer typically will make all the decisions for you). As long as you are a part of the decision-making process, you will be strengthening your navigationals skills. In London cab drivers, they've discovered that their brains expand as they aquire 'the knowledge' of the city: http://www.bbc.co.uk/news/health-16086233