I've played with Airtabs on my Audi wagon. Did a few A-B-A test runs, didn't see any worthwhile improvement. I only had the mounted with blue painter's tape, stayed on but easy to remove. Ended up returning them.
For over a year I have been curious about AirTabs on the vans (Sprinter, PM, Transit, whatever). Figuring that being a larger and longer vehicle, the effect of AirTabs would be more noticeable. I've even come close to ordering another set for testing.
But then I saw an older Sprinter with them installed. It looked goofy. Downright oddball. Approaching ugly. Since I expect any mileage gain will be a small percentage, I decided that I'm not willing to give up the looks for a wee bit better mileage.
Now if you're doing 100k/yr on the road, maybe they'd work for you. I'd still look at doing some A-B-A testing to see if they're worth it. Go do some test runs on a flat road, no wind, preferably a few miles long. Tape on the AirTabs, do some more test runs in same conditions (same day, preferably). Then take the tabs off and test again, you should get the same numbers as the first test. Thus, A-B-A,.