Well in the interest of full disclosure, first I must mention that I got my knobby knife free. I called the owner and we had a lengthy discussion about his product and the outcome was a free tool to do a product review. So, here goes.
See the link at the bottom to view the product in use.
I received my Knobby Knife about a week ago. My 450 has a new tire, so I tried it on my 250. I've sharpened knobbies many times before using a couple different methods. I've used an angle grinder. It worked, but takes off a little more than ideal. I've used a razor and torch. That method is accurate, as far as how much you want to remove. However, it is cumbersome and time consuming. I'm not going to review how a sharpened tire works becuase I've ridding those for years and I'm convinced it is a great idea. Basically, if you're going to keep a dirtbike tire more than about 6 hours of riding, sharpening you tire will give you much better traction for the rest of the life of your tire. I don't think anyone will argue with that.
Now to the knobby knife. Mine looked a little different than the one in the video and picture. The handle is made of wood which probabbly insulates heat better. It also has a metal clip around the neck to keep the blade from touching anything. According to the inventer, the knobby knife is 100 watts instead of 60 watts like many soldering irons. The purpose of this is to maintain 500 degress. This allows you to cut knob after knob without waiting for the iron to reheat. I found this is absolutly true. It also has a nice wide blade that is perfect for a knob. I was able to sharpen my tire in less that 15 minutes and I believe next time it will be under 10 minutes. Unlike the video, I recommend using gloves and needle nose plyers to pull off each tab you remove. The precision of the knobby knife will allow you to sharpen your tires multiple times. The key, I believe, is to limit the amount you remove off the width to maintain a constant width to hight ratio (i.e. knob strength).
In summary, I found the knobby knife is an outstanding tool. If you keep a tires near the amount of time as an average recreational rider, the knobby knife will give you better traction. In addition, you will likely keep your tires longer because they will work better longer. The knobby knife gets an official CincyDirt 5 stars out of 5 stars.