As luck would have it, we have a typhoon today, and the wind and noise woke me up at 3am, so I snuck downstairs to get a little sharpening therapy.
I won an ebay razor that was practically shave ready, but since it had some chipping, I decided to bread knife and start sharpening from scratch. I just got my JP 4K and 8K stones in the mail, so I thought I'd give them a try. (I also got my Naniwas, but that is for another thread:))
When I was told about the stones, I figured it would be a hybrid - fast cutting like the glass, but with a sturdier binding like the pros. Turns out my assumptions were true. As mentioned, the JP stones are gray, while the regular glass are white.
I used the 1K and 2K glass, then tried the 4K's - a few passes on each to feel the difference, if any. Personally, the white 4K responded more favorably than the JP on the razor. The JP 4K felt very similar to the pro 5K, but with just a little more "give" (Olivia knows what I mean;)).
The 8Ks were another story, though. I actually found the JP 8K to have more positive feedback tan the white 4K. Again, the 8K JP was similar to the 8K pro, but with a little more give. (The 8K pro is actually softer than the 5K pro, which is a very dense stone.)
As for the purpose of the JP stones, it is quite clear that if one doesn't have a Lie-Neilsen plane, the JP glas stones are a more cost effective alternative (or better, compromise) to the 15mm pro stones.
Pics and product #'s below.
:D