So it seems there are two versions of that lens out there.. Very good (from which the "nifty fifty" got its reputation) and a bad one (maybe made in who knows where).. I can not believe the people who were so happy with their specimens did not know what they were talking about.
See also here:
http://www.astrosurf.com/buil/50mm/test_us.htm
At some stage I was wandering myself should I go for it or not, and then this FD popped out.. based on test photographs taken with similar lens designs, I have made the decision and took a risk (because at that time I was not entirely sure how the mod will go) and now I see it was a correct one.
Ash, if you can not find a good specimen, do as Jeanette.. However it should be at least as good as kit one.
And then perhaps try one of those M42 lenses on ebay.. with adapter it will cost you not much more than $50, and no or very little coma :-)
And manual mode only, of course.