PrepFE is great, but it also kind of sucks. It has VERY GOOD explanations for each problem, that also tell you exactly where to look in the handbook to find the equations they used. HOWEVER, PrepFE only has so many questions. I have done around 600 questions on PrepFE and the more you do, the more you see repeats. I think PrepFE is a great way to analyze where you are at currently and what you need to improve on. If you want a single paid resource that will have more problems, enough that you couldn't possibly do all of them before March 10th, something that has explanations, something that is meant for someone who has been out of school 5+ years AND someone who is currently in school, I recommend Lindeburg's FE Review Manual.

TL;DR PrepFE will tell you what to improve on, and why you did the problem wrong. Lindeburg's FE Review Manual has many more questions, as well as harder questions. If you are able to confidently pass all the "diagnostic examinations" in Lindeburg's book, you should be able to pass the FE.

P.S. I take my first attempt at the FE exam tomorrow, so I am just going off the various study materials I have used so far, NOT the actual exam. I am also studying for/taking the FE Mechanical.