I've had them on my last two trucks.
Advantages: Can hitch up and not worry about the gate. And yes I did forget to lift my gate once and cut a huge gauge in the 5er on the first turn.
While it doesn't help mileage. it does decrease turbulence. With my solid gate I had a blue tote behind the hitch that floated. with the louvered v-gate no such floating.
disadvantages: Not intended for any type of load. You can't step on them and you can't slide anything of real weight over them while loading. Can't use a bed cover to secure cargo. According to Stromberg Carlson the plastic cannot be painted to match your truck.
If looking at used ones check that the latches are steel. early ones had plastic latches that failed. SC replaced them with steel to solve the problem.