I've also started to research car mounts pretty heavily for a couple of reasons. One, navigation and easy access for other tablet tasks I might need while on road (focused and within reach of me as driver), but also Two, for my kids to YouTube and NetFlix from the rear seats.
Due to these needs it seems the best options are either the floor (bolt that connects to seat) mounts seem to be the most effective or cup holders. Something tells me the cup holders won't hold it securely enough while in motion but also are typically geared for driver/passenger need and wouldn't be high enough to view from the rear.
This led me to believe the air vent mounts or suction mounts would be most ideal for all needs but again, tend to be less firm in motion and also don't allow for actual interaction with the tablet as pressing it will probably dislodge it.
VERY interested in staying up to date on this thread, anyone with any suggestions and/or recommendations, please chime in!