They are extremely important. Right now it is probably the only objective, though imperfect, way of assessing medical knowledge. A score of 260 corresponds to about 90 on the old 2 digit scale. The higher your scores, the more competitive your application. Also bear in mind that your first passing score stays with you.
Unlike MCAT, GRE, GMAT you do not have the luxury of multiple attempts at the USMLE to improve scores.