Via Usability News, Usability of GPS Receivers in a Sporting Environment by Rodney Sloan and Jacques Hugo(Department of Information Science, University of Pretoria):

In spite of the growing popularity of GPS, there are many usability issues to be addressed. (...) We looked at learnability and discovered that many GPS devices did not include even simple help menus. As we all know, most users read a manual only as a last resort! Most of us expect the device itself to answer our 'how to' questions. (...) the input systems employed had a low utility as it takes a significant amount of time to input data into the device. (...) In terms of personal safety to the user, giving attention to the device while performing specific tasks may be hazardous in the same way as driving a car while talking on a cell phone (...) What is not easy to memorise though is the data stored on the device, particularly waypoints, which could become confusing with large sets of data.(...) Knowledge of the buttons and controls for a specific device is needed, as is knowledge of the display. The user also needs a basic technical understanding of how the device functions. For example, it is important to understand how the GPS receives satellite signals and how it calculates the heading of the user. In field studies, users who did not understand these points became frustrated and complained that the device was not working properly.

I think ths most interesting critique is this one:

Most conventional GPS receivers are specifically built for the outdoors, with waterproof sealing and sturdy, drop-resistant design. These features suggest that the designers have taken some of the things the user will typically have to face into consideration. However, much more attention should be paid to the context of use, which includes a closer look at the variety of environments, specific user tasks and interaction modalities.