I have Kens 6, 3, and 1 micron diamond plates. They work well - just be careful with them.
I will say that I was surprised how fast they work the first time I used them. I used soapy water with them, as I do with the rest of my EP stones. I put about 3 drops of water on them and spread it with my finger. Then I strop with edge-trailing strokes. The water that collects on the blade side of the edge starts to get dark from the grinding quite fast. It took me very little time to go through the 3 levels.
This is on 3V steel too. I couldn't get my EP stones and polish tapes to work worth a crap. I could get a cutting edge on the blade, but it just never got to a real sharp edge. The switch from the 1000 grit EP stone to the 6 micron diamond plate alone is night and day - and they are essentially the same micron level. I don't know if this difference would be present with some lower grade knife steels, in that the lower quality abrasives (aluminum oxide, silicon carbide) can have the same micron level as a diamond abrasive, but the difference I saw is that the diamonds are much better at wearing the hard 3V steel (vanadium carbide in the steel - the aluminum oxide and silicon carbide stones wear down the other elements in 3V OK, but the vanadium carbide stays as it is a much tougher element to wear off - the diamonds will take this at the same time the other elements, so you get a much finer edge).
Depending on what steels you intend to sharpen, and what finishes you expect to get, you might be OK with the EP stones and then switching at the 6 micron level I did. The regular EP stones are adequate for bringing an edge up on the steels I have. I like a real sharp edge so at the 6 micron level where I switch over to the diamonds it works just fine. I just don't have a diamond finish in any more coarse levels.