Fits very securely. Dropped off only once when I left it parked outside on a very hot day, but reattached once cooled. Because the holder fits inside the vent surround, the back of the unit is tight against it and so no jiggling even when jabbing the screen to enter addresses etc. As mentioned, not sure of the name of tape but nothing special - double-sided white foam tape that i got at local office supply shop - possibly 3m. You could use the stuff that holds on number plates also. Any possible residue can always be cleaned off the vents with a dab of petrol/white spirits/wd40 if you wanted to take it off eventually.
Model is Tomtom Start (10) I believe. Its an older model with the smaller squarish screen, so not sure if it's only available secondhand now. I've attached a (stock) picture showing the nav with the holder part that fits inside the vent surround.