I am still using the solar panel, although with newer GPSs you don't need the external NiCads. This is because the external power input (from the solar panel) both powers the GPS and charges the GPS's internal lithium battery. Without the NiCads, it is necessary to regulate the output from the solar panel, however. Older GPSs like the GPS III could not charge their internal batteries from the external power source, hence the need for the external NiCads. NiCads, while being old technology, are better than lithium batteries for this purpose because of their tolerance for the widely varying charging conditions caused by the solar panel.
What am I using now? I still use the GPS III in areas covered by the base map. I use a Foretrex 201 for other areas. I am thinking about splurging and buying a Quest so I can have detailed maps just about anywhere I go.