I have to disagree about the "too little too late", Mark. While you most likely would not change the finished song, it would always be valuable input for the next song you work at.
And, as for myself: I remix my old songs every now and then, and it ALWAYS helps to know what people REALLY thought about the previous version.
If we only give kudos to the folks whose songs we comment, we need not comment them at all. That's useless, because it means nothing. How can the artist improve if he does not get honest feedback?
It's very different to get feedback at the production stage than after a song has been finished, btw! While you are working on the song, you are much more biased and may not want to accept criticism, albeit constructive and right. You just can't hear it, so it must be wrong (psychological thing - we can't fight that). When a song is done and you are working on other stuff, you might have a more objective view on the finished song and listen better to what other people have to say.
Regards,
Steffen