I don't trust myself to know - while I'm writing code - how well I or anyone else will understand it cold. So if I'm in any doubt at all I leave a comment. Sometimes I do go back and remove them - refactoring has to include comments.

Whenever we discuss this topic there are usually some defensive sounding remarks about the fact that inaccurate comments are much worse than no comment. This is certainly true but I don't share the conclusion that the only reasonable answer is to never leave a comment. I guess if that is already what you do, then its probably easier to conclude it is correct.