I'm lead dev for a large website which has a higher than average percentage of IE6 users - about 4.4% of our audience. Our new version is going to make use of progressive enhancement - including transitions and effects as well as rounded corners, gradients, web fonts and other CSS techniques.
Obviously there are cross-browser ways to achieve most of these things which require various amounts of work to implement.
What I'm currently looking into - and what I'd like your experiences of - is how to decide at what point we draw the line between providing an enhanced experience vs just supporting the functionality.
FYI, I believe that this question meets the six guidelines for great subjective questions as defined in the FAQ. I'm after answers detailing why and how, not too short, with constructive comments, experiences, facts and references.