Bought one off Ebay from China for a few bucks; took a few weeks to get here, but I gave it a try last week on a mango tree. It's very crude, basically a bent blade (poorly) soldered onto the shaft. Works fine on green or thinner bark, but it's difficult to use on thicker/more mature branches. Blade kept getting clogged with bark and it wasn't going deep enough. I don't think it can handle much force. I gave up and went to using my curved grafting knife, much easier.
It's useful, just don't expect too much from it.