I can confirm everything MikeF wrote (good and bad things), and BlueHorse is wrong (at least about the kind I have -- maybe his is a different brand, though) in that the fan does work more in brighter sunlight: although you can't real feel it, you can easily see the fan (inside) turning faster or slower depending on the sun's brightness.
My wife swears by it (she, not me, uses it because she has to park at work in a lot where her car heats up)!