I have had the the bowen single edged belt buckle knife. when you draw the knife, the belt is unsecured, and ceases to hold up your pants. I wore mine all the time when I lived in tampa, but I didn't need the belt, as I liked my pants tight around the waist.
It can be somewhat fast with practice, but I found it to be clumsy overall, especially if the belt is too long. when the knife is in the sheath and the belt is fastened, it is quite secure, as in it won't fall out.
The knife itself is curved to contour to the waistline and is somewhat thin. I found holding it was sort of uncomfortable, and I have small hands. I could not get a very secure grip on it, plus, it is stainless steel, and could be slippery.
I always thought of it as a very last ditch self defense weapon.
It is also considered illegal in many places so keep that in mind as well.
Pete