[Nick Griffin]"Does anyone have any experience using Google Earth Pro for video output? "
Yes. It exports it to any QT format. If you have FCP on the machine with Google Earth Pro, you will have access to ProRes.
[Nick Griffin]"How hard is it to program different rates/speeds of zooms? "
You can't determine the speed of the zoom. At least not that I have seen. It will zoom in much like the regular one does. Faster the further away from the area you are. So if you want a slow one, start closer.
[Nick Griffin]"Can you pause mid-zoom and then easily return to a zoom in or out? "
Not in Google Earth, no. You'd have to do that in your NLE.
[Nick Griffin]"How difficult is it to use / how much practice does it require to get good at it / or is it simple from the outset?"
VERY easy to use.
Also note, you MUST include the MADE WITH GOOGLE EARTH in the lower right hand corner...that is already burned in. You cannot crop it out. That is part of the agreement with Google Earth to allow use of this in a broadcast show. Check on the licensing fees. At the time we used it we had to pay $500 a year for use of it in broadcast shows, on top of the cost of the Pro version. Not sure if that changed.
I will recommend do a zoom in or zoom out on after effects based on pictures from google earth. Every time you do a zoom in there's always bad pixels. You can check andrew Kramer's tutorial about it. Easy to follow and way better results because you can rotate the camera while you are zooming in or out, add clouds and birds or what ever you need. I hope it helps!