I guess I will be the lone voice of decent.
I do not use or recommend them.
The horizontal holster violates the second safety rule (never point the weapon at anything you are not willing to destroy). The muzzle of the pistol is constantly pointed at whatever is directly behind you. Then, if the snap comes loose, gravity will take effect. I have seen this happen in restaurants, on the street and in the sand. The vertical holsters are somewhat better, as the muzzle is pointed down.
There are a few valid applications where hip, IWB, or thigh holsters will not work (e.g., riding a motorcycle / bike). Another application is for security drivers (for their second / backup gun).
Finally, these holsters are mechanically inefficient. Try reholstering with one hand… heck; it is still somewhat awkward using two hands.