We use google earth to measure properties almost exclusively. It's too time consuming going to a property, measuring and looking at the terrain for just mowing or fert.
The key to success for us has been knowing our service areas well. We know what areas (in general) are relatively flat and what areas have more slopes, etc.
Knowing this had made Google Earth work well for us. It probably won't work in every part of the country, but it does for us.