We have a plastic one but that was because we bought a used truck that had damaged tailgate and it was cheaper than a new OEM tailgate.
Overall I like it. It's nice when you have to pull it off because it only weighs like 10lbs.
Big trucks and towing mean you get lousy MPG. I wouldn't count on a noticeable effect towing or not.
I also wouldn't count on being able to just leave it up and ignore it while hitching. Ironically, the prior truck had the standard tailgate. Never came close to hitting it with a couple hundred hitchings. First time with the new truck and the V-gate the pin caught on it while pulling out from under the hitch. Luckily being plastic it just flexed and popped out of the hinge. If it was metal, it would have been permanently bent and in the trash.