Very pricey piece of beauty... Never owned any, but have seen many results. Very sooth and aesthetically soft wide open lens. Its a pre-war design, so of historical value too.
The difference with the Helios is that the Biotar is sharp wide open. The Helios probably makes a better general purpose lens, but I have an 85L so I'm not looking for that.
I believe my fat version is post war and the thin version is pre war. But perhaps I have that reversed.