I have added one.
Fuel economy impact? Unmeasureable. Too small a difference to tell.
Handling? I have 20,000 pounds of downforce, almost 7000 pounds in the front. If the airdam makes another 50 or 100 pounds, I'll never know.
Biggest impact? Cooling. My fan used to come on quite a bit, even at highway speeds in normal, 80ish, weather. Since I added the airdam, 7.5 years ago, I think the fan has come on about 10 times, maybe. Never on the highway. It REALLY stabilizes the coolant temp.
Quoting myself from a 2007 post about my airdam addition:
I added a 15" x 8' airdam under the radiator last week. Drove to Florida from Detroit last Saturday. Ambient temps never above 75, but water temp NEVER went over 191 degrees! It's amazing to me that Ford doesn't put any airdam. Perhaps Winnebago's combination of grill opening and hydraulic leveler placement ahead of the radiator hurt my specific coach, but it would seem an airdam would help no matter what the various coach builders do. Workhorse puts an airdam, take a look at the next time you see a Workhorse chassis.
I posted pictures of it somewhere, can't seem to find the post now.