Yes, it does depend on where you are. It depends on the state and area. The blury images you see are satellite images from a LANDSAT IMAGER most likely. The detailed aerials are images taken by plane, like in Indiana (my home state). Local and state governments can decide when to sell the rights to Key Hole or Google Earth to display the images. The images from Indiana where a product of Homeland Security and therefore had to be made public.
In almost all cases, aerial images in either .tif (mrsid) or jpeg files can be downloaded with spatial references. For example, I have Pike County, KY aerials, the same exact ones on Microsoft Local Live (USGS Orthoquads) and all the USGS topo maps loaded on my computer. I simply add them as layers into ArcView or ArcExplorer (GIS) and I can pan and zoom quickly and easily to locate trails and such. You can use other programs to make route layers that then could be loaded into your GPS. This information, with the right plugin, could all be overlayed into Google Earth.
So, in short, yes, it does depend on where you live, but with the right tools, you can add a lot of power to Google Earth.