I need to read the original site because possibly some of this information is already there :-) But, cooking has always been a hobby of mine and I fell in love with Japanese knives when I moved to Japan 7 years ago. Here are some answers to your questions. I'm not an expert so I hope I don't make too many mistakes.

No western equivalent: Some Japanese knives are definitely completely different beasts than western knives. For example the nakiri knife is only sharpened on one side (the right hand side, away from the thing you are cutting). So it's a bit like scissors -- perfectly flat on the left hand side and sharpened to 22 degrees on the right. This allows you to get a much cleaner cut to vegetables and is absolutely necessary for some of the decorative cuts that some Japanese chefs do. It actually requires a different cutting motion to use -- you draw the knife towards you as you slice down as apposed to the western technique for pushing the knife away from you.

There are some other very specialized knives for cutting fish (which he does not have on his website). One in particular that I can think of (the name escapes me) is practically a sword, being very large and heavy. It allows you to easily slice bones very thinly and hence make them edible (fish bones are a big source of calcium in Japan).

As for the santoku being a different knife, I have to say that I find it more similar to a french chef's knife than a Gyuto so possibly I'm missing something there :-)

Magical Construction Technique: Do they really have a magical construction technique? Amusingly, the answer is very close to "yes" ;-) Western knives are often cast steel that are then shaped and sharpened. High quality japanese kitchen knives are made the same way as a katana. They are folded time after time after time which aligns the carbon in the steel making it more stiff. This allows the blade to be sharpened better. However, the metal is very soft so you need to "wrap" the metal in a harder metal. This is what is known as a "kasumi" knife. When you sharpen it, it exposes the layers of different metal giving you a really pretty pattern on the blade. Although the pattern in pretty, that's not its function. It is to provide stiffness along the blade so that it doesn't bend or warp. Whether or not this is desired depends on the type of knife and what it is going to be used for (this is where I rapidly run out of knowledge, unfortunately). In addition to grinding through various layers of steel, a very good kasumi knife can also exhibit mottling from the carbon in the steel similar to damascus steel.

Do you have to do this for 100 years to be able to make a good knife: Obviously not, but think for a moment about the number 100. What is true 100 years ago that is not true now? The answer is: they were still making katana 100 years ago. Katana were still made for the samurai class until the late 1860s (the Meiji restoration). After that, they were made for military officers until the end of WWII. The technique for folding the metal is unique to Japanese sword/knife manufacture and is incredibly labour intensive. Slowly the artisans that were making swords 100-150 years ago have been migrating to kitchen knives, scissors and wood working tools. Most of these manufacturers are family businesses -- just some old guy and his brother (and maybe one of their sons if they are supremely lucky) working out of a shed. My personal opinion is that high quality knives will be hard to buy at all 20-30 years from now because most of the people doing it now will be dead. This is not factory work -- these knives are crafted by artisans using techniques that are not well known. It's rather sad that it is hard for these artisans to excite the next generation into taking over the business (I think mainly because they make practically no money doing it :-( )

Carbon steel knives are dramatically better than stainless steel for use. Even a cheap, mass manufactured carbon steel knife costing $20 will beat the pants off of a stainless knife for a very simple reason: it is easy to sharpen. It is true that it does not hold the sharpness for long (and it may be my post that you are referring to). General rule of thumb is to sharpen it every day (after you are finished using it so that it has time to rest before using it again -- most chefs have 2 sets and use them on alternate days).

The other main benefit for a high quality carbon steel knife is that the flex of the knife is designed into it by the artisan. Because they are folding the steel, they can get a lot of different effects depending on what they are going for. There is a huge variety of Japanese knives for this reason. The "this has no western equivalent" should really be taken as "this is designed for a specific usage and has characteristics which would not be possible using different materials/techniques". Whether or not you need such a niche tool is really dependent upon what you are doing, though. (although I think the santoku is a wonderful general purpose knife).

Once you get used to using (and sharpening) carbon steel knives, I pretty much guarantee that you will also think of stainless steel knives as not worth the effort. It just isn't possible to sharpen stainless steel to the same degree. The only other knife that is comparable IMHO is ceramic (but which is completely unsharpenable and is only good for a couple of months depending on how much you use it). My wife prefers ceramic over carbon steel because it is very, very light. It's a bit less versatile, though.

There are numerous disadvantages of carbon steel. You have to sharpen it very frequently (every day). You have to keep it clean. You have to keep it dry. You can get rust on your carbon steel knife if you even so much as look away, I think. Japanese chefs are obsessed with their knives (and cutting boards) and you have to have the same attitude.

Although the website may not discuss the downsides of carbon steel adequately, I don't think they are incorrect about the overall message. Stainless steel will give you a crappy knife that is easy to look after. Carbon steel will potentially give you a fanstastic knife that you must care for.

IMHO he is also correct that Japanese carbon steel knives are unique in the world (I don't know anywhere else that makes them the same way -- I'm interested to hear of any, though). In particular the knives he is selling are hand made artisan knives. Far from being able to find similar knives in other places in the world, the knowledge (and desire) needed to make these knives is even dying out in Japan!

Hope that helps. Like I said, I'm a big fan of Japanese carbon steel knives, but I'm not an expert, so please forgive me if I have made mistakes in the above.