I generally only use the GPS on my phone when I'm driving from one state to another. I know my city and I never use it while driving around town, but today I decided to slap on the dongle and take my Prime for a spin to see how well it tracked me, and to test some other things.
I used "Navigation" from google. Typed in my destination while still on a wifi network. Then I went out to my car and drove. I didn't check while driving (that would be stupid) how many satalites were tracking me, but it seemed to follow me well, always knew where I was, followed the turns immediately. I specifically drove off course to make certain it could recalculate without a data connection. It did without a problem and rather quickly, though beiing a short trip and near my destination I don't think it would have much of a problem.
My concern is, though the trip only took about 15 minutes (in retrospect I really wish I had timed it more closely), I ended up using 5% of the battery, which seems to give it about a 5 hour use time. Granted I had the tab in "Balanced" mode and the screen brightness on minimum. Navigation of course disabled the screen sleep mode and I imagine one could extend use time by turning off the screen (as voice directions will still activate).
Still it seems that the extention kit does drain the battery, as is warned right on the unit. "Unplug unit when not in use to save power"