A slightly biased view I think. A significant model in the Contax history but not one to consider using today. They are unreliable and difficult to repair. I've repaired a number of them but, today, I won't touch them as they are too much trouble. And most of them were used by professionals and got hammered. If you want an RTS, get the RTSii. A much better camera, more reliable and repairable.
I currently have nine RTS bodies, none of which work and I have three RTSii bodies of which two work and the third is repairable (if I ever get around to it).
The 139 is a good choice. It can have issues but is repairable. I still have my original one I bought in 1980 and a couple of others. I have one on the bench now going through a full service before being put up for sale.