I have the Koishi. and a Denka on the way (scheduled delivery......tomorrow) The Koishi is a good blade, I like the steel, it seems to take an edge pretty easily, and had good retention. I can usually go most of the work week with only a daily stropping at the end of the day. As someone mentioned above, the Koishi grind is slightly concave. The shoulders on mine were slightly eased by a previous owner, and I have gone a bit further with that. While it is thin behind the edge, I have never found it delicate, as it has a sort of thick spine and some taper coming out of the handle. It feels pretty robust.
Once the Denka comes, and I get a chance to use it a bit, I will check back in!