I decided to try using proportional fonts for coding starting a year or two back. It worked out well and I stuck with it, because proportional text is easier for me to read on the whole, and because it allowed more characters to fit comfortably on each line on average. I did find after a while that occasionally the lack of alignment between characters on two subsequent lines was a problem, but then I configured my editor so that it showed comments and text strings in a monospace font and that fixed the problem for me.