First, bear in mind that everything I am saying is second hand, I haven't used a CBN wheel.
From different manufacturers they are available in various grits from 80 to 220 grit, perhaps others but I haven't heard of them. One warning, according to the threads I have read on the AAW forum, these grits aren't comparable to stones for deciding what wheel to get. A 180 grit CBN wheel cuts as quickly as an 80 grit stone according to reports.
Another consideration is weight. Although some brands are lighter, the eight inch wheel I am interested in is nine pounds! Six inch wheels are roughly half the weight of a comparable eight inch according to one supplier but he didn't have a real number when we were talking on the phone while he was on the road. All are heavier than stones in my understanding. Since you don't need guards all the way around CBN wheels I am considering putting a six inch wheel on my eight inch grinder just to not turn as much weight.
The CBN wheels do have a short break in period, again according to reports. I would expect a very small change in size during this period. After that there is very very little change in size so you can mark the fixtures and jigs you use and basically forget about set-up time. At least one brand of CBN wheel has grit down one side of it for I think about an inch on the eight inch wheels, has the grit on the six inch wheels too but no reports of how far down it goes. Since the heavy aluminum body on the CBN wheels won't break or shatter like a stone there is no risk to side grinding on a near perfectly flat surface. I don't know if the wheels with grit down the side have square or round corners, my guess is square so you might have to consider your usage and decide if round corners or grit down the side is of more value for you. The selling price for all wheels from the company with the side grit is the same but to keep the price the same they don't offer the 220 grit with the abrasive down the side according to a supplier. Perhaps it just isn't offered here, I don't know. The sizes are originally metric on the wheels from Austria I believe, other than shaft sizes I assume, so widths and diameters are slightly different than stated in inches.
The CBN wheels produce very little heat or sparks. Little heat is a nice thing, few sparks means you have to watch very closely to see when sparks are coming over the edge and the tool is sharp when doing pretty large revisions.
A nice thing about the CBN wheels is that they are universally reported to run very true from the factory. The ones from Europe, I think Austria, are wider and heavier than the ones from China. I have to admit, unusually in my opinion, the wheels from China and Austria appear to be equal in quality for all practical purposes. I don't know which is cheaper when you consider the amount of grinding surface on each one. With the life most of us will get out of any CBN wheel it is probably a nonissue.
I have been looking at CBN wheels for about a year now, from the time I started turning. They are nice, but there are some pretty nice stones for thirty to seventy dollars on up to a hundred or more. A fraction of the price of CBN wheels but yet another advantage of CBN is the amount of abrasive dust they don't put in the air. One of those things from working in welding and fabrication, I hate abrasive dust. I hate thinking I always get a little in my lungs and I hate thinking at least a little is getting on everything around where I use a dry abrasive stone. I'm sure there are still some particles off the CBN wheel in the air and breathing in the particles from the steels is none too good for us either but CBN wheels vastly reduce the amount of abrasive in the air and of course eliminate the binder used to hold the stone together.
That's pretty much all I can remember from my reading on CBN wheels and a bit more than I know!
Hu