What schools are you applying to? If you're looking at schools ranked in the top 50 or so, then most of the post I've written below will probably apply. If not, then take my opinion with a grain of salt. And that's all this really is, is my opinion based on my own experience with the admissions process over the past year.

"Low" GMAT score is a somewhat relative term depending on school selection. Generally anything at the bottom or below the 10th percentile of GMAT scores for a given school is low (probably bottom 25-30 percentile is more like it).

For top 10-15 schools, I think anything below a 680 or so is considered low. Anything below 650 is probably "non-competitive" for a lot of top 20-30 schools. A 590 is more than likely two standard deviations below the mean for most top ranked schools. Assuming schools admit roughly 600-1000 people per year (depending on class size. HBS admits more than Stanford, for example) probably less than 15-20 people per year are admitted with a sub-600 score (this is an unscientific number, mostly a guess on my part. But you can be assured that the number of admits with a score in the 500s is vanishingly small). And these are likely people with extremely compelling stories, careers, grades, and a blue-chip recommendation or two in their file.

If you're planning to apply to schools in the top 50 or so I think you have a lot better chance of gaining admission if you spend your time and energy on studying for and re-taking the GMAT rather than an optional essay. If you can even get from 590 to the mid 600s, you'll probably increase your application's value by a wide margin, a much wider margin than even a well-written optional essay will.

UC Davis Rankings:
Poets&Quants: 58
Forbes: 68
U.S. News & World Report: 42
Financial Times: 80
The Economist: 60

Median GMAT: 700
GMAT Range (mid-80%): 650-740

I would also suggest that you retake GMAT and take it up to least 650 to add value to your application.
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