I think that nobody likes to receive a negative comment or critisism on their work. However, a lot depends on how things are said. You can tell your opinion in a polite and constructive way or you can really slam the person. I think the first way is more likely to make the artist at the receiving end think about what was said, mull it over and maybe realize the truth in it and learn from it. But the second way would make most people become angry and defensive. I agree that if you post something as a WIP you should be ready for the critisism and negative comments, but I suppose most people hope that these will be helpful and not hurtful.