While I have not taken the GRE using ProctorU, I have taken a few online classes that used ProctorU for the exams and can share my experience with it.
To take the test with ProctorU, you have to have a computer or laptop that has a webcam and strong internet connection. I remember having to show the proctor, my laptop screen by holding it up to a mirror, having to show all four corners of the room, and my sitting arrangement. In addition, they initiated remote access of my computer so they could check for any programs etc that might be running. Once that is all done, you can sit down and take the exam.
Personally, I hated this experience as I was much more conscious of the fact that someone was watching me the entire time, and I was extra cautious of where my hands were at any point to ensure that they proctor did not think I was cheating. Also I had to worry about where my wife was at any given time because you were not allowed to have any other person in the room with you. Honestly, if given a choice, I would prefer to take the test in a testing center as ironically I would feel more comfortable and relaxed there vs at home with someone watching me the entire time.
Happy to answer any questions or clarify anything. Best of luck to those taking the test.