As far as fuel savings, I never really saw any advantage that could be deemed as savings. With weather conditions different on each trip it would be hard to determine cause and effect for the device.
I have and use a second hand deflector on my 5er. At a cost of $60 I could justify it at least to myself. I had a diy deflector mounted on top of a shell at the rear with my old TT setup. I felt less turbulence with the TT, the stability increase with a SRW truck and TT made it worth having. With the 5er and a dually truck stability is not an issue.
As stated above the kamikaze critters miss their target more often and have to look for another battleship. The only downside is the surviving bugs are still able to reproduce.