Don't know about Penn State diamond tools but diamond stones in general last a very long time. THey are much harder than the different types of tool steel we use.
If you're thinking of getting them for lathe tools, I would back up a bit. There is a debate (soon to follow I'm sure) about using tools straight off the grinding wheel or honing after grinding and between grindings.
My experience after making a lot of bowls and learning from Stu Batty is that using a tool straight off the grinding wheel gives an incredible finish if your technique is good. Honing takes extra time so why bother. You should be able to start with about 180 grit (150 on really problematic woods) after using a sharp gouge off the grinder.
Some people like slow grinders, others fast. Some use water (Tormek); some use under the wheel systems (work sharp, jool tool). I have tried (and own, sigh) all of them.
I finally bought an 8" variable speed delta grinder. Not too expensive, excellent balance. Most of the grinding wheels are designed to perform at high speed (3500rpm or so). So I use mostly at the high speed. I use mostly 10v tools, so I spent a whopping $100 per wheel for the super sg wheels from Craft Supplies -- BOY, what a difference. And they do last a long time. But it's an awful lot to pay for wheels.
Anyway, these are some of my thoughts.
YMMV.
Dave