I explored this, and even ordered one off amazon (though may have been a different brand) - decided it was too bulky in appearance and didn't suit the car, and was also put off by reported lag issues and difficulties in making sure the phone was correctly paired at the time that you needed the camera the most.
It was mainly the reported connectivity issues that ended up in me returning it - perhaps if you are willing to go the hardwired route it might be a more reliable solution.
Would be interested to know how you get on if you do buy it, can't deny the convenience/safety aspect.