Adjustments are just from semi-loosely threaded fittings. They're tight enough to hold it in position, but you can manipulate it's position pretty easy..
In it's position now, there's just enough room to shift comfortably where your hand doesn't hit the top when going into 5th or reverse. I does pretty much eliminate the usefulness of the middle seat for anyone over 7 years old, though.
I used 1" pipe for this, I think that 3/4 would probably work better, and be cheaper.
I haven't got around to getting GPS yet, but I *think* it will work great. I have used it to let the kids watch DVD's, you don't have to take your focus too much off the road to see the screen. I also ran an aux input wire from my stereo up under that seat so I can plug into the headphone jack on the laptop and have "surround" sound! It's GREAT! (I don't think you could hear the crappy laptop speakers anyways..
Bush -- you bring up a good point.

I had NOT planned for a rapid deceleration! After installing it, I drove around like a maniac trying to get it to slide around to test how secure it would be. Under normal conditions, it doesn't move. But you are correct, in a crash there is nothing to secure it to the top. I had thought about using two strips of velcro around the top of the laptop and screwed into the top to do that. When not in use, you could hook them up underneath the base to keep them out of the way.