It's been a few years, but I'll try to remember. These used to be one of those "unfair advantage" parts used in cost no object motors that had to meet the letter of the law, meaning no roller lifters. They weren't roller lifters but they used roller cam profiles giving the same advantage.
I don't recall exactly how much they cost,maybe $1500? Don't quote me on that, it was the first figure that popped into my head. Back then(1999) you had to buy them as a package. You sent them your cam, and they would have it coated.
There is a bit of "cool" factor to them, though. They feel like porcelain, but seem to weigh nothing! Spring rates could be lower to free up a little power.
That might be the one real benefit for a street motor, lower spring rates. Otherwise, rollers are "cheaper" and easier to obtain.