Electro Deposited Diamond Stones 
Diamond sharpening stone have been around for many years. The DMT stone consists of a steel backing plated with electro deposited diamond grit in a nickel matrix. The diamond bearing surface is backed up by a sturdy glass reinforced plastic base holding the plate stiff and flat. Unlike regular bench stones which can be used up like a bar of soap, the DMT stone has only a single layer of abrasive and there for a limited working life. As with any product, diamond sharpening systems have their advantages and disadvantages. They are not well suited for sharpening steel for even the hardest steel has the ductility to degrade the diamond abrasive much like a fingetip degrades a sugar cube.
Diamond sharpening works very well in spite of this disadvantage. I've used them for many years to keep my bench chisels and plane irons keen and scary sharp. Restoring an edge with a DMT stone requires certain technique which appears in the enclosed directiond for use.
They are subject to abuse and their surface may be stripped of abrasive if the sharpening process is forced.
I'm ready now to replace one of my 5 year old DMT stornes that has gone bald.
Simply the best.
This is a great product. A medium grit and fine grit combination DMT stone are all that most people will ever need. It may seem expensive, but consider the fact that once you set the edge angle with the medium side, you can usually polish it in minutes to a razor edge. Two things: 1) get either a video or an instruction book on HOW to sharpen - there is more to it than just dragging the blade across the stone. 2) periodically scrub the surface of the DMT with soapy water and a stiff brush, this will clean out the deposits in the surface and allow the diamond deposits to cut properly.
I am puzzled by one reviewer's assertion that these are not suitable for sharpening steel. I used a DMT stone for 8 years to sharpen a dozen heavily-used restaurant knives (all Wustoff or Globals) and since then I have used the same DMT for 3 years to sharpen forged Japanese woodworking chisels (Rockwell hardness of 62-64 - which is far harder than most knife blades).
I haven't noticed any loss of sharpening ability at all and still put a shaving sharp edge on knives with it.
If theoretically you could wear off the diamond deposits, it would still be a better choice than traditional "natural" sharpening stones. While it is true that you can continue to use "natural" stones like a bar of soap as they wear down, that is one of the worst things you could do to your knives, short of using them their edge to hammer nails or something. That's because the key to quick, proper sharpening is maintaing the correct, consistent angle of the blade edge against the stone. When a natural stone wears down and become concave, it is almost impossible to maintain the correct sharpening angle as you move the blade along the concave (not flat) surface (It is difficult to maintain the correct, uniform angle even with a perfectly flat stone or diamond hone but that's another story). So people who are attached to "natural" stones and know what they are doing periodically use another stone to grind down the surface of their sharpening stone so it is flat. That is alot of work.
Finally, I've dropped the DMT a couple of times, once on concrete and once on tile and it was undamaged. Many "natural" stones are so brittle that they shatter or just crack apart when dropped.
