Yes I have one. It has some good points, and some bad ones. I also have a Copperdog tray that is vastly superior in nearly every way. I have used both quite a bit so I feel I can give you a fair and objective review.
Zero Gravity (ZG) vs Copperdog (CD)
The Ultimate Tray Shootout!
Construction of the two is very different. The quality of the ZG is that of fair to good consumer grade, using mediocre materials. The quality of the CD is commercial/military grade, using only top quality materials. No compromises. No comparison.
ZG harness uses a waist band which is the only thing I like better about it than the CD, it keeps the controller from swinging around when you need to bend over and use 2 hands. Although the waist band is one more thing to adjust and connect, making the CD easier to get on and wear.
The CD is vastly easier to mount an dismount from the controller. One clip and it's secured or freed, it doesn't rely on the tiny little loops on the ST16 face. A real PITA to connect to.
The ZG has no true retention system other than a pinch-friction fit clamp at the base of the ST16 for it to slide into. You can pinch this together for a tighter fit. However if you move in the wrong direction too quickly it can slide right out. It is still connected to the little loops so it doesn't fall, but it frees itself from the harness and swings around your neck until you re-position it the friction grip again.
The ZG is very comfortable to wear, but VERY complex to adjust to the sweet spot, takes a lot of fussing around to get all the different adjustments symmetrical for it to hang at the right height and the right angle. The CD only requires you to adjust the height, the angle is already perfect (for me).
The only attraction of the ZG is the price point. The ZG is about the half the price of the CD. However as soon as you hold a CD tray in your hands, you doubt the value of the ZG. As soon as you mount the ST16 and put it on, you know the CD is the ultimate tray and well worth the extra buck$.
I have not used my ZG since getting the CD. The ZG has been relegated to the 2nd ST16 I only use for Team Mode. I wear the CD and let the cameraman fuss with the finicky ZG.
If you really can't afford a Copperdog, the Zero Gravity is the next best alternative. However it trails the Copperdog by miles.
Disclaimer: I was not paid by Ty Pilot for this endorsement. ...I was threatened!