I've not owned one... but I'm considering a fixed 16 or 20 for 600-1000 yard steel, and have read nothing but praise for the SS line of scopes from SWFA and if I go fixed, that will be my choice.
They don't have a 12x, but 10X is more than adequate at that range, in either the rear or side focus- and less $ than the Leupy. Worth a look.
http://swfa.com/SWFA-SS-MOA-Scopes-C4226.aspx