Naniwa Superstones are fantastic. I purchased the #400, #1000 and #5000 and I have absolutely no regrets. In the last month I have sharpened butcher blades, pocket knives, hatchets and kitchen knives.
Before I purchased them I had read lots of reviews, watched lots of videos and even talked to some people with knowledge about them. I wasn't sure how well they would hold up as some people said they were soft and don't last very long. I have to disagree with that. The only thing I can say about them thats really a negative is they do load up a little faster than others. Easy fix, rust eraser. Couple seconds and you are back in business.
The #400 cuts fairly quickly and does a gives the knife a very nice edge. It also gives it a nice polish. If all you can afford is one stone I'll be honest the #400 will get the job done. You can repair a knife edge and sharpen it. The #400 is my go to, and it's the stone I start with just about every time.
The #1000 is a great stone. For some of the knives this is where I stop. It leaves a great edge and polishes the edge quite nicely. The #5000 is very fine. Leaves the knife with a very nice polished edge. (I'm sure the higher grits are outstanding at this however). I don't use this stone all the time. But I definitely enjoy it.
The Naniwa Superstones are a great quality stone at a great price. The polish you get from them, with the speed they cut and how they feel. You get a lot out of the stone for a fair price. I have no complaints about the stones. I love them and will be getting more of them.