I have one in my Suzuki Grand Vitara (please don't tell me this is an SUV, and not a real 4x4, I know
) and I use it the whole time, although I have tinted windows. I always have "something" in the trunk, sometimes of no value (now helmet and safety shoes), but sometimes my travel luggage, including my computer. It is also my way of limiting my travel luggage: everything should be hidden by the trunk boot cover.
Thieves look in the car, when they see things, they break windows and steal, or don't steal when it is of no value, but you have a broken window. When the trunk cover is used, they don't see anything. But of course, they could think: "there is something under the cover".