Chris Accetta , Apr 15, 2005; 01:46 p.m.
Used wide open the 58/1.4 is a very slightly softer than the MD 50/1.7 I also own. The difference is neglibile, though (it's only 1/3 stop). The usual caveat applies: your results may differ.
To me, the bigger drawback to the 58/1.4 is that it's a little more flare-prone. I should probably use a hood for shooting in bright light.
Hope this helps. Enjoy the new lens.