I only took it this October. Definitely should have allotted myself more study time and taken it more seriously. I scored in the 46th percentile (domestic student, if that matters).
I'm applying to applied math programs, and almost all (90%) of the schools say that the subject test is "optional, but recommended." My highest reach school, Stanford's ICME, says "It is highly recommended, although not required, for Ph.D. applicants to take the GRE Subject Test in Math, Physics, or computer science."
My application is fairly strong, I think. I have a great GPA, good general GRE scores, solid research experience in the subfield I'm interested in studying. Two of my reference letters are from well-known profs in my field and the third is my undergrad academic advisor. I'm pretty sure my score detracts from my application entirely, but I'm not sure what the admissions committees will infer if I leave the score off.