In my view, the NBME is a useful tool to use earlier on in assessing standing and progress. I took form 1 before studying and was then able to go into studying with a bit of confidence (be careful to not be too confident and slack off). Then I took form 5 three weeks before my exam. The NBME's are of limited value as predictors because you cannot know what sort of questions will show up on your exam (e.g. mine was very evenly distributed and was micro and pharm heavy, whereas I've spoken to a classmate that had little micro and pharm and his exam was heavy on nutrition and GI pathology). I think the utility of the NBME is primarily to acquaint you with the format of the questions, the range of question types that may be posed (i.e. the level of integration is at a somewhat lower level than USMLE World, in my view), and to give you a range of what sort of score you might expect (i.e. if you get a 230 on the NBME a few days before the exam, you're not likely to get a 270 or a 190 on the real thing). That said, I would not spend too much time shortly before the test doing multiple NBMEs - that time can be put to better use by reviewing the sources (especially FA and path) and perhaps cramming a subject that wasn't studied before (neuroanatomy and biochemistry in my case).
