I have used one occasionally. They're really intended for deep concealment, such as when you have to take your jacket or coat off and need some way of carrying. I'm referring to wearing a dress shirt or something similar where the shirt must be tucked in.
There are two primary drawbacks, though. First, reholstering is not easy. Secondly, drawing from concealment is slow, but it beats not having your gun.
FWIW,
emc