What do you know and how do you like the RCA 5963 Black Plates?

I just rolled a nice set of 1957 RCA 5963 black plates with foil D getters.  I don't have many hours on them yet, but I was surprised at the sound quality and value for the price on the used market.  

I don't know much about this tube, other than it being a substitute for a 12AU7.  What do you know about them and how do you like them?

Thanks!