I have had one for about a year now, shooting through a 3d season, and a hunting season, and now into a target season. Last Feb. was the first time I ever picked up a bow, so keep that in mind. It's the only bow I have ever shot, aside from the ones I tried at the bow shop I got it from. Just to put it out there, not that it matters to this discussion, I am in the top three in my class at local 3d shoots, and I usually shoot 280-290 at our local league shoots, so I don't think I'm a terrible shot.
I bought a Quivalizer because when I was researching what kind of setup I was going to get, I read online that for a hunting bow, everyone needs a stabilizer, and everyone needs a quiver, among other things. So when I read about the Quivalizer, I was excited that you could get both, in one device, for less weight that two separate devices. I had to have it, because it only made sense. My rig is pretty light, it's a bare bones Carbon Air 32, and I was trying to keep it as light as possible.
So in using the Quivalizer through the seasons, I can say that except for in my back yard, I have never used it in the stabilizer position. It is always in the vertical position. I feel like the extra weight so far forward makes me grip the bow differently, and I end up adding some torque and\or heel to my grip. I have shot with it out to 70 yards in both positions, and I am more accurate with it in the vertical position. I could probably add a back bar to balance it out, but my whole point is to be as light as possible, so that is counter productive.
I can also say, that when hunting, I would never put it in the horizontal position. I hunted last season both in Northern Alberta, in what I would consider fairly open country, and on Northern Vancouver Island, in what I would consider very non open, steep country. It's hard enough with just a bow in your hand, but a bow with a Quivalizer (or any other stabilizer stuck out that far) it's damn near impossible to hike through the bush and not get hung up to the point where you start making a ridiculous amount of noise trying to go around things. So it stayed in vertical mode the whole season, in two provinces.
As far as target shooting goes, I am very much a 'train how you play' type of guy, but even so, I take it off most of the time. I have shot 290 with it on, and I have shot 290 with it off. It doesn't seem to make very much difference. I just put 3 arrows in the hammer pocket of my pants, lol.
The clamps do rattle if you don't tighten them when they are empty, and the hood (I have the original with the foam) is not deep enough for my broad heads to not stick out. I'm shooting 125 gn Strickland Helix. They stick out a good 1/4 inch, which is for sure enough to get cut, and can also cut clothing. I know this from experience, not just because they stick out. The arrow gripper is pretty good, it grips my deep six XD shafts just fine. You might have a problem if you are shooting the super skinny shafts though.
