This is not a gadget but a necessity.
As I only shoot with primes. I did acquire the Capture Lens as soon as released and it works wonders for whomever shoots with one body and 2 lenses. Is is really fast and practical as I used to carry a second lens in a pouch.
It also defeats the infamous camera gear thieves as they are at loss trying to unmount the lens.
The longest lens I use is the 105 E with the hood on and it rest easily on the hip. You need to tighten your belt though.
What I like is the fact when crouching or seating. You rotate the lens towards your groin and it stays safe and comfortable even with the hood on.
There is another company which started a Kickstarter for a 3 lens mount which I am also interested in.
You need not have the mount on your belt as it can go on your backpack strap or wherever.
The only caveat when on your belt is it digs into your belly but they have a special Pad for it.
Here is my well used CaptureLens: