I asked a guy that had one on his truck and he claimed that he gained about 50 miles per tank with his previous 5th wheel, but not as much with his present travel trailer as the distance between deflector and trailer is too great to be effective. He had his mounted on a bracket right behind the cab.
Shortly after seeing that deflector we took a trip and I paid attention to how many semis have deflectors, and I'd guess about 90% or better have them, usually integrated with the cab. Now obviously there is something to be gained with those kind of numbers with that widespread usage, and I found a test report online that came up with about 15% savings with a complete aero kit on a semi (both cab and trailer). Now keep in mind that a 5th wheel is more aerodynamic than a trailer to begin with, a deflector for our trucks will not match the efficiency of those semis' integrated designs, that the savings only occur when you tow, and your mileage may actually go down with a deflector when not towing (even with one that can be lowered, due to the increased weight).
With that said, they look cool, may help with the bugs and stability especially if you don't have a dually, and may even save money if you primarily tow with your truck and put on a decent number of miles. For example, 50,000 miles at 12mpg and diesel for $3.50 is about $14,600, so saving about 10% or $1,460 would probably pay for the deflector.