I recently purchased the Boker koppo, which arrived in the mail yesterday. My first impression was one of solidity. The Boker koppo, although it has a hollow core giving it the shape of a test tube, is made of solid steel.
The advantage of the koppo over the kubotan/yawara is that the kubotan has a finger loop. This finger loop enables the user to open the palm for grabbing or striking with the open hand. This means that the koppo functions much like the little-known palm sap.
My one complaint is that the finger loop is too big and too loose, but it should be a simple matter to remove the current loop and replace it with a shorter length of paracord.
You can read another review of the Boker koppo here.