I got these in December. Unfortunately, I'd more or less finished packing up my shop to move, and it's taken this long to get them back out and give them a fair test.
So here are my thoughts.
Where, and how much?
These are available a lot of places. At the moment, you can get them from Amazon (here) for about $75. They may be more or less expensive other places.
What do you get, and what's the construction like?
This is a kit of three diamond plates in a fitted wooden case. The plates are 325 (blue), 600 (red), and 1200 (green) grit diamond mesh plates on a plastic substrate. The wooden case is decent, and well enough fitted to prevent damage. You can use the stones wet (with water) or dry.
How do they work?
Quite well. Up until I got these I'd been using an old combination oil stone; medium on one side, fine on the other. It worked well, and still does, but oilstones are comparatively messy. They must be oiled, and they tend to stain if you make the mistake of putting them down on cloth or wood.
I've done three real sharpening jobs with these.
First, the standard stuff. Chisels that I'd gotten moderately sharp, then used until they were dull. I skipped the coarse stone here, and did the 600 and 1200. I always follow with a leather strop charged with some kind of yellow stropping compound. I've lost the packaging and don't recall the details, so that's all I can tell you. These worked great. Taking curls off wet Douglas Fir end grain great. I'm pretty sure my technique hasn't gotten that much better, so I have to assume the plates are a lot better than my old oilstone.
Second, a first-time setup. I had a plane that I bought unused, but never did more than a cursory swipe over the oilstones. So it had the factory grind and a little more, but had never been what you'd call "sharp". I started that one on the red plate, but moved back to the blue when I figured out how flat the bevel wasn't. Once the bevel was sharpening as if it was fairly flat, I moved to the red and green plates, followed by the strop. This particular plane isn't what you'd call a precision tool (it's the Shelter Institute rabbet plane I reviewed last year), and I'm not very good at adjusting it, but it works a lot better now than it did. It still needs some tweaking, though.
Finally, a damaged plane iron. I have an old Stanley #5 that I use for stock removal. It wasn't good for much else, because there was a big nick - maybe 1/32" deep, maybe a bit more -- I couldn't be bothered to remove from the edge. I had a good #4 for smoothing, why do I need a #5 set up for it? This was the real test. The iron is exactly the same width as the plate (about 2"), and had been indifferently sharpened with the nick in it. So I started with the blue 45 grit plate. I still don't have the nick completely out, because I only had about 10 minutes to work. It's faster than the 80 grit sandpaper I've used for grinding, and leaves a much nicer surface. I followed it up with 20 strokes each on the red and green, then a few on the strop. I also ground the back on the blue plate and polished the back on the green -- it's not quite mirror finished now, but it's pretty close, and the nick is almost entirely gone. Total elapsed time on that blade, maybe 8 minutes. It takes nicer shavings, too.
I'm not going to tell you to rush right out and buy a set of these. They're nice, no doubt about it, but there are some limitations. If you have a system you like, and it gets your tools as sharp as you want them, stick with those. Don't give in to the new tool temptation.
So what were the limitations? The main one is size. These plates are 6"x2". That's fine for chisels, pocket knives, and rabbet planes, but it's a challenge on anything big. For the #5 that I was working on, I had two choices: skew the iron and run it across at an angle, or use my fingers as runners to make sure it didn't slip off the edge. With my try plane (an old Siegley 24" transitional), I won't have that choice: it uses an iron about 2 5/8" across, so it will be skew or nothing. If you don't sharpen any big tools, that's not a limitation. For me, I'm considering buying a larger stone or plate just for my try plane.
Would I buy another?
If these vanished, I'd probably go get another diamond plate set. That said, I think I'd look for a larger plate. I'd like one large enough that I can sharpen any of my bench tools without having to skew them, and that means I need a stone at least 2 5/8" wide. More than that would be better.