The last knife I purchased is an Spydie Endura 4 with the ZDP steel. It is very true that it's difficult to put a edge on it.
Usually when a knife comes from the factory, there's a little more refinement that can be done to make it sharper. Using a DMT Fine/Extra fine sharpener, I've tried in vain to get the edge razor sharp. (It was hair-popping sharp from the factory).
I use the paper-cut test. Using sheets of copier paper recycled from work, my test of sharpness is to get it to the point where the blade will shred with virtually no resistance or "catching" on the edge of the paper. When this becomes effortless, you know the edge is pretty dang sharp.
This usually takes me 5 minutes with softer steels like 8Cr13Mov or even 154CM. This ZDP-189 has been murder to make a dent in, even with diamond grit.
The only issue I have with the Endura is the handle being a bit narrow for most hands. The thinness of the handle though makes it a good EDC, even though the blade is 3.75", usually I like an EDC to be closer to 3".
Also, I find the lockbacks to be a little stiffer than liner locks on deployment. But the Endura is the least expense knife with ZDP-189 that I know of.