No offense to you for wanting one or anything, but I've always thought they look silly. If the sun's high enough for them to be useful, the interior sunvisors work every bit as well (particularly on RAV4s, I've found, as they fit very close to the rearview mirror), and if the exterior visor's down far enough to be effective in any other circumstances, it obstructs field of vision across the top of the windshield.
That, and it also seems like it would disrupt airflow and cause some issues with gas mileage, being noisy at higher speeds, and aerodynamics in general.
My personal opinion of them is roughly the same as my opinion of adding a spoiler the size of a DC-10 wing to the back of a Honda Civic. I'm not saying that it can't be, or isn't, done, but that it's pretty much unnecessary and that the money could be better spent on other, more nifty alterations.