They definately work, but sometimes, you need to be in the "sweet spot" to feel the effects. What this means is the V2 redirects airflow, above the rider, or at least higher than stock. If you are in the sweet spot, you will not feel the full blast. But also keep in mind that air speed around the sweet spot will be higher, so it could be worse than stock.
They are especially useful in the racing circuit where a stock windscreen is not tall enough for the rider to tuck underneath the airflow. By having a taller screen, the rider can get completely underneath, gaining a few MPH. This is why you will never (or hardly ever) see anything other than clear on racebikes.