It seems like whenever I spend a long time on a painting, I become completely blind to it and can't assess its strengths and weaknesses. This is such a source of frustration for me...I often find that I'll re-work an area for hours, only to wipe it all off and start all over again. But I'm so fortunate to have a couple of great sources for critiques. My husband is not an artist and has never had any art training, but he is blessed with an astute, critical eye and often can spot things in my paintings that I just don't see. I rely on him to come in the studio at the end of a long day of painting and see my work with a fresh perspective. Sometimes he tells me things I don't necessarily want to hear (like "it's not finished, is it?" or "is that the block in?") but I know I can count on absolute honesty from him, and that's the most valuable thing any critiquer can give you. An honest negative critique is better than a dozen with empty praise. My other source is my artist friend Terry Pappas (see her work at www.efgallery.com )- she's a great painter and is very skilled at spotting what a painting needs. Even though she lives 2 hours away from me, thanks to the internet and jpegs, I can send her an email and get a long-distance crit when I get stuck on a painting. Thanks to both of you!

via web
As for critiques...if constructive, they certainly can be useful. However, sometimes the pièce de résistance evolves from learning when to stop.
I remember when I was in a class at the Academy in Chicago doing and re-doing an acrylic, when the instructor came by and lifted the canvas I was working on. "Hmm," he pondered, "Are you considering selling this when finished?" "Yes, I might," I replied with a smile. "Well, you might consider selling it by the pound to recover the cost of the paint!" I quickly lost the smile, and thought twice before over "re-doing" a painting.
I no longer paint, as I now live vicariously through my artist clients, and create gicleés of their work. I'm still applying that "knowing when to stop" theory to my fine art reproduction.