They say they don't look at writing, but I wouldn't at all be surprised if some admissions officers just glance at it. But don't get me wrong, the CR/M traditional score is still the most important SAT Score.
Its not like each section is printed on a separate page or anything so unless they are physically covering up the writing score with their hand, they are going to judge the score. A 500 will certainly raise some flags and an 800 will probably be a big boost (the admissions officers are in fact human).
I got a glance at my student profile on the monitor of my major (AEM)'s dept head and there was CR, M, but no mention of WR. She also mentioned that she reads all of the essays so I assume she's also doing admissions
At the A&S information session I went to 2 summers ago, someone asked the woman who was giving the presentation this question, and I specifically remember her saying that they don't, in fact, place as much weight on writing as the other two sections.
That being said, I got an 800 on writing, and a 680 on math, so I would suspect that those two kind of balanced. Then again I did get an 800 on the CR as well, so I'm sure that had something to do with it...
Overall, I would say it depends on what school you're applying to, and what field you might be interested in. Despite what the A&S admissions people say, writing probably matters more there than at the other schools.
As the poster a few above me said, if you get a 500 or an 800 you will stand out, for better or worse.
I'm of the persuasion that it really all depends on you're other scores. I did relatively ****ty on math, but my other scores made up for it. Had I gotten a 720 in CR however, I may not have gotten in.
Let's put it this way, a really good score on writing is not going to hurt an overall lower score - the overall composite number is available to admissions, so they are at least, going to glance at it. The writing brought my daughter's score within a decent overall range for Cornell. It probably also depends on your potential major. Hers was not math related so the higher writing offset the lower math. I believe in her case for CAS, it helped.