I really like Google My Maps - its possible to produce a map without needing a PC powerful enough to run Google Earth. However, if you try and get your data out (to edit further in GEarth say or mashup with other data) you are linked to a network link. This opens the data in Google Earth and will automatically update any change to your Google My Map but isn't any use if you want to extract your data - it stays on Google's servers.
Barry Hunter explains a workaround here. Thanks Barry!