Seeing as there's probably a few people reading this with Borosilicates, are they worth the extra price?
(ps, I'm mostly doing landscapes with slow lenses f/6.8 or worse wide-open, if that makes a difference).
Hmm... not sure if my setup applies, but no one else has replied. I needed new ground glass for a Horseman VH 6x9 view camera. I went for the Hopf borosilicate. At that size, the price was around $50, as I remember? Compared to about half for the plain glass? Nothing like a jump to $250 or so.
It is nice, a 'cleaner' white than the old Horseman screen. Maybe a touch brighter. Just a touch. The real factor that affects the ground glass is the fresnel, as i see it. If you have a top-notch fresnel already and some extra cash to get rid of, the Hopf screen would be good. Worth it? I don't regret it for my 6x9 Horseman, but I wasn't looking at a $100 plus price difference.
Put your money into a fresnel, then decide on the ground glass, is my advice.
As to putting one on a Rollei SL66, I think you would be better off with a pre-made fresnel screen. Plain ground glass in a reflex is dim. There are many options for the Rollei that don't involve making your own fresnel addition. Also, aligning a focus screen in an SLR (or TLR) is often not a simple replacement. On my Horseman, the ground glass surface indexed to the frame and film plane and it was simple drop-in.
I don't know what screen came in your camera, but by the time Rollei was making the SL66 their screens were pretty darn good. You might be spending a lot of money for little improvement. Here's a comparison I made for TLRs. As you can see, the plain ground glass is in a league of its own, and the Hopf borosilicate wouldn't be significantly brighter...
http://www.rangefinderforum.com/forums/showthread.php?t=142212