It might vary a little from school to school obviously. Some schools use a point system for admissions, others are more subjective. From what I have heard about my school from faculty and the admissions committee, the essay is very important--people here have had very creative essays and I have heard the Adcoms mention that so-and-so impressed us by saying ______ in their essay.
There are several essay writing resources available, so if that is a weak point make sure you take advantage of them (even if it is a free service offered by a writing lab at your school). But also remember that a few school will ask you to write an essay when you are being interviewed as well.
Good luck!