I have a pair of them that I've not yet tried. They're for a 5E3 build.
I recently did a bit of mucking around with new manufacture rectifiers, and found that only the Shuguang hit the numbers of the old Mullard GZ34 I have. Neither the JJ nor the Sovtek made it. They tested the same as an old 5V4. Details here.
A little while after that I came across this interesting little read from a bloke who claims to have been involved in the development of that valve. It's very interesting, if it's true.