We got one because we bought a used truck with a damaged tailgate and found one for $60 on craigslist.
- Old truck with standard gate: Nothing flew out of the bed (we had empty 5gal fuel jugs for around 15,000miles of towing that sat quietly), never felt anything, never came close to hitting the tailgate (it was put down when hitching)
- New truck with V-gate: We've lost a few small wood blocks to wind and a cooler half loaded. First time out we hit the V-gate (luckily it is plastic so it flexed and popped off the hinges without damage). The V-gate now goes down.
I would say it's a******shoot if there is a benefit and certainly wouldn't count on it for MPG gains.