The built-in rear-view camera system in the Leaf provides not only the view but dotted lines providing depth, with the lines shifting as the steering wheel is turned. Very nice! I also opted NOT to get the center display console with my iMiEV as I thought it was overpriced.
In one of my EV Sparrows I have an add-on rear-view camera with the image displayed on the radio screen. It's better than nothing, but provides no depth-of-field information and I am unable to judge just how close I am to a nearby object.
One of the reasons for choosing the iMiEV is it's better all-round visibility than most of the vehicles out there (especially SUVs), and I don't feel a pressing need for a rear-view camera; however, if someone can come up with an add-on which provides a good undistorted view with depth and turning information, I would also be interested.
Of greater interest to me is an add-on side view camera to supplement or replace the driver's-side mirror and its very restricted field-of-view (legally mandated to be flat and not convex). http://wheels.blogs.nytimes.com/2010/12/30/in-defense-of-convex-drivers-side-mirrors/
Hmmm, perhaps the two can be combined into using a single display by the left A-pillar?