I've tried it a few times, but it's just so painful when you want things to align. Now, if you had a really good code layout tool that could 'snap' parts of lines so that they horizontally align at sensible places, I think non-monospace fonts could be useful.

In fact, having smart alignment would be great even with monospace fonts. I work in emacs and it's frustrating when the editor decides to 'fix' your own spacing because the editor-mode doesn't have a very intelligent layout system...