I own one of these from my brief foray into the Contax world. For 135mm, it's completely useless:
Yes, these ponies were supposed to be in the centre of the frame! I suspect that they couldn't push the 135mm frame far enough to one side to accommodate for the parallax as they were somewhat hampered by the position of the RF patch. But for 85mm, it works really bloody well:
I actually quite like it for 85mm - it makes it much easier to focus and frame quickly, and the view through it really isn't bad at all by 1930s standards. I'd never use it for 135mm, though; not only is it wildly inaccurate, but the tiny box you get look through is even worse than a VIOOH stopped all the way down. The edges aren't so well-delineated, and the RF patch takes up 1/3rd of the view. It's just not a good time.
The only other thing I'll say is that while the 543/7 doesn't really affect the (already bad) ergonomics of the camera, taking the damn thing on and off can be bit tough. Don't take it off with an 85mm lens mounted or there's a good chance you'll gouge the barrel of the lens as it slides out (that's a mistake you only make once), and be careful how you push from the back as the Contax's accessory shoe can be vicious if your thumb knuckle hits it with some force (damn thing cost me some blood first time I tried to use it).
Needless to say, I agree with
@dexdog on this one: Zeiss made some great lenses, but the people designing the cameras definitely didn't think about the end user!