i also have a symmar 210/370. i have shot it both as a 210 and a 370 ( portraits and architecture - color & black and white). it is true the lens it weighs a ton, but considering all the stuff i have to carry - it really doesn't matter too much . is sharp in the short focal length, and it has a nice softness to it when converted. mine has weird black specks inside the rear element ( separation?) so it might be softer than the usual lens ...
i really like it because it kills 2 birds with one stone, AND it covers a 5x7 negative without a problem.