Read the bits about feedback (and feedforward) in Marshall Goldsmith's book What Got You Here Won't Get You There
For a capsule version - watch his google talk (search on his name plus "leading at google").
Watch for the danger of "trying to add value".
That said, I have written brutal but really in-depth critiques of someone else's work with great fear and received effusive thanks and praise for my efforts. I've made what I thought were mild suggestions and eventually was told that the person couldn't sleep that night because she was so upset. None of these were people over whom I had any power at all. The first one was my boss and the other was a friend.