This knife is not sharpened on one side of the edge only , like a chisel, a Yagagi bi or many other traditional Japanese knives, as the first reviewer stated. Maybe he meant it was not a double edged knife, but I don't think any kitchen knife is. This is a Western style Japanese knife, but still has the harder steel and small angle bevel of Japanese influence. It is a double bevel edge. Most production knives are sharpened on belt sanders and buffers and the edge may be barely perceivable as being asymmetrical. Maybe that is where he got that idea. If it is resharpened, other then with the crock stick or steel, it should be sharpened on Japanese water stones and given a double bevel of 15 degrees per side, 30 degrees included, unlike German knives which are usual 20 degrees per side or 40 degrees included. If you are not familiar with that you should go to a good professional sharpener who is familiar with Japanese knives.
As far as cutting the sponge, try laying a wet washcloth down on the edge of the sink and wipe the blade off wiping away from the tip. I understand what you mean though, it is sharp!
I have the MTH-80 Mighty Chef Knife with Dimples. It came extremely sharp out of the box and stays that way. I have only honed it lightly a few times with a borosilicate glass steel after smacking the cutting block with my poor style. This thing rocks. With my Chicago Cutlery knives the same abuse would put very slight dents in the edge barely visible as a glint of light under a bright light and I could feel the loss of sharpness after a few cuts. The edge on the MAC is so acute and fine that I didn't want to take a change and loose anything with a steel (which is not recommended anyway) or even a 1200 grit ceramic steel (which is recommended) so I went with the borosilicate glass rod which is finer. It still cut fine before steeling the first time and I didn't notice a difference in cutting until I steeled it and then I could feel it cutting better. The rod brought the edge back to new or better every time. This thing is tougher and holds an edge better then good non-stainless carbon steel knives I have used.
This knife is thinner, lighter, the steel is harder, it holds a edge better, and is more nimble then a comparable Western knife like a Wusthof or Henckel. It cuts most things you cut with a chef's knife with less force. Some may not like that. I do, although I wouldn't recommend using it like a clever and you probably could get away with that with Wusthof or Henckel. They both have their place, but I like the MAC for basic slicing and light chopping better. I think you should be prepared to sharpen any new knife before use if you want a really sharp edge. This is the nicest cutting knife I have used. It is also the sharpest out of the box.
Some have recommended the Forschners from Victorinox . I tried the paring set for $10 and the serrated parer or sandwich knife. They are sharp but not as sharp as the MAC's. Also, the Fibrox handle, at least on the paring knives, is so small it gets lost in my hand. I cut a hard crusty French bread with the serrated knife and the points were showing signs of blunting or rolling over. A little steeling brought that back but I don't think the Forchners are all they are cracked up to be. I would agree they are probably the best knives for the money (for example those 3 knives for $10). They are pretty amazing for the price but the MAC's are one of the best cutting experiences you can buy. You can pay more for something to look at but the MAC's has the same performance.
Update: I got the MAC PK-30 paring knife and the SB-105 bread knife to replace some of my other inferior knives. Those 3 (plus maybe a Santoku) are really all I need. I love them all.
The MAC's are not real fancy with silver inlays or high polish or anything but it cuts like a razor and stays that way better then any of my other knives. The fit and finish is good too. As for the chemical stain I think you should expect that with any high carbon or very hard stain resistant blade. This is made from one of the super steels. It is about all out cutting ability and you have to take care of it a little.
The steel, edge angle, finish, and sharpness is very similar to the Globals but I don't like the feel of the Globals and it seems like the handles on the Globals would get slippery when wet or greasy. I like wood or micarta for the handles. Also, the MAC came very sharp out of the box. They say Globals are that way too, but some I looked at in the store had a burr on the edge and need some attention before use.
To keep it sharp, I would recommend anyone with a knife like this, the Shun's or the Global's either get the MAC or Idahone ceramic steel, an Idahone 2 or 4 stick sharpener, or a Spyderco Sharpmaker. Amazon has the Idahone listed a "not available" but you can get one at Sonoma Cutlery. The Sharpmaker also has the 15 degree angle available and is sold here on Amazon but is hard to find. Do a Google search on "amazon spyderco sharpmaker" it is at http://www.amazon.com/Spyderco-204MF-Triangle-Sharpmaker/dp/B000Q9C4AE if they let me show the URL. These are easy to use and you get the correct angle just by holding the knife vertical and stroking. the 4 stick model also has a 15 and 20° angle for the sticks. I would use the 15 degree slots first and later you can touch up with the 20° for a micro edge before having to do a serious re-sharpening.
I don't usually come back and rate something I bought, but I did a lot of research and after using the knife through Thanksgiving I have become passionate about it and MAC knives in general and want to share them with everyone else. They are kind of a secret and fairly hard to come buy in the non-commercial world. Also MAC's are not sold in sets and you may not even be able to build a set from one style of MAC (like Classic, Professional, Superior, etc). You may have to mix styles. This is apparently in Japanese tradition even though these are Japanese made western style knives. What you get with most Japanese western knives is Japanese steel and Japanese style sharpening and profile. To make a knife this thin work with such a fine edge you have to have harder steel then western manufactures generally use or the blade would quickly dull from use.
For other references check out Cooking for Engineering Chef knife test.
(do a Google search on it) and also see Knife Forums. For KF do a Google search with the knife you want to explore with the KF in the search. For example search " best chef knife site: and put knifeforum. com ". Leave off the quotes and spaces. Sorry if that was a little cryptic. It looks like Amazon won't let me put the URL's here.
Update 12/2013: Eventually I did a major re-sharpen and it is still going strong. The three Mac's I have are now my favorite knives in the kitchen.