The problem with knives---and with blades generally---is that regular use requires resharpening. Our forefathers back in the Bronze Age learned the vast superiority of metal to stone. But the more expensive metal required maintenance and, as can be imagined, bronze and copper blades weren't all that durable. No doubt many of the old-timers back then argued against the innovation and proudly clung to the tried-and-true stone implements (which, despite their crude appearance, actually took some skill to make).
Things went decidedly for metal blades with the advent of the Iron Age and tempered metals were born. From the early cast-iron models to modern stainless and titanium steels, tempered metals held a sharp edge like nothing else before or since. Files were used to sharpen them, as well as the carbine stone.
So, a few years ago in the knife-making center of Theirs, France, the Arno Company made one the first innovations in sharpening tools:
which combines the effectiveness of a carbide stone with the mechanics of a file. The Arno sharpener comes with two heads: one for sharpening and the other for burnishing.
Having used this tool for several years now, I can attest that Arno's sharpener lives up to the company's reputation. One of the best things about the Arno is that it will work on any blade---from ladies' pocket-knives to machetes and axes. It doesn't require different sized stones for different sized blades. Also, it doesn't require any set-up equipment. Just hold the knife in one hand and sharpen with the others. It also works surprisingly quickly.
A few internet trolls have written negative things about the Arno. One is that they allegedly tear edges and ruin blades. That can result from any improperly-used sharpening tool. In every comment thread I've read claiming blade damage, the naysayers admit to not reading the instructions. Others claim that they leave 'jagged edges'. That can happen too if you press too tightly and the blade chatters.
One of the silliest statements about them is that they are intended as a 'last resort' tool---i.e. you urgently need a sharpener and nothing else is available. I cannot imagine why anybody would buy one for such a purpose. Any theoretical emergency kit could hold a traditional stone or file just as easily. If either is so much better, why keep an Arno at all?
My suspicion is that much of the animosity coming from so-called 'experts' comes from the fact that Arno and many similar products are exclusively sold; and that many of the critics have an interest in those companies' competitors. As far as I know, the Garrett-Wade Co. of Brooklyn is the only outlet for buying Arno's. The sharpeners cost around $45; but since I last bought one, I see that Arno also now offers a model with a sharpener alone for about ten dollars less.
A very useful tool. Some guys I know own several---one in the toolbox; one in the kitchen; one in the camp-kit, etc. They're a great investment for the price.