I use U.S.E. Ring-Go-Star on both my hard tail and road bike.
It does what it's supposed to do. However, the way you adjust your stem will be backwards. By backwards I mean this. With a star nut, or a compression cap, I first preload the compression cap until the headset doesn't have any play anymore, then I align the handlebar and the fork, then tighten the stem.
With the Ring-Go-Star, you put the Ring-Go-Star between the headset and the stem. if you have some spacers, you put those on top of the Ring-Go-Star. Then you align the handlebar and fork. Then you push the stem down the steerer tube as tightly as you can. Then tighten the stem. Afterwards, then you adjust the Ring-Go-Star to get rid of the play in the headset. The last think you install is the cover over the stem where the top cap normally goes.
The Ring-Go-Star works like this. When you turn the bolt on the Ring-Go-Star the lower ring expands. Since the Stem is already tightened, the upper ring stays and the lower ring pushes down and loads the headset.
On my setup, The Ring-Go-Star takes the place of a 10mm spacer. So I get rid of my compression cap and a spacer. Yippie!
On my mountain bike, I don't put the top cap. I can look though the hole and look at the tire.
Once it's setup, it works real well.
U.S.E is quoting 25 grams for it's weight including the top cap. If you don't use the top cap, you get so save some more weight.