If we are honest with ourselves, we would always
BUT
Your critique partner, that's who.
Everyone needs at least one. Sometimes
What is a critique partner? A CP is someone with whom you trade manuscripts
This is not to say that your critique partner can't be a cheerleader, but the biggest, most important qualification when seeking one is honesty. Are they willing to be honest with you so your work will improve, even if it might make you temporarily unhappy? If they write in the same genre as you do, will they allow a competitive spirit to cloud their judgement? Face it: you don't need someone to tell you how he would have written it. You need that person to tell you how to improve what you've already written and called your own. They should be able to sprinkle praise in the middle of it all, but if the feedback is all at one end of the spectrum or the other, it won't be nearly as helpful.
Critique partners are a gentle way of thickening your skin for the not-nearly-as-kind reviewers out there. If no one has ever read your work prior to publishing, or if you've only ever had attaboys from friends and family, then that first negative review just may crush you.
Hey, it's even biblical: "Wounds from a friend can be trusted, but an enemy multiplies kisses" is right smack dab there in the middle of Proverbs (27:6, if you don't trust me and want to look it up). You need to be able to trust the person to love you enough to hurt you, which is an odd concept, but honesty from a friendly voice is always easier to handle when you know that person risks much by their honesty.
Critique partners can be many things, but I'd say "invaluable" is the best way to describe them.
Do you have a critique partner? More than one? Never heard of the concept? Do you go through them like they're disposable? I'd love to hear your thoughts.