If you have the postwar lens (either the Zeiss-Opton T, Carl Zeiss T or East German Carl Zeiss Jena T), then you have a very fine lens.

The postwar f/1.5 Sonnar is single coated, so you still need to take care when shooting backlit subjects.

There are a number of us who shoot with the lens and really like it a lot. I'm not sure how many use it wide open -- I tend not to want to always shoot wide open, though I have used it in this manner.

In general, it's a very sharp lens with good contrast and very good resolving power.

The lens is limited in its ability to shoot close-up. Without any adapters, I think minimum focusing distance is just under 1 meter or 3 feet. That's typical for a rangefinder. Even at that distance, you run into parallax issues because of the offset viewfinder window.

The f/1.5 Sonnar is a good all-purpose lens. I've probably shot maybe 50 or 60 rolls, mostly b/w with some color transparency and the occasional roll of color print. I've had very few complaints with the performance of the lens.

You can see a handful of shots on my own Contax IIa mini-site.
http://host.fptoday.com/melek/zeiss/contax_iia.html