I passed the Security Plus exam today with a 825/900! I believe that if you have already acquired the Network plus cert, you're chances of passing it will greatly be increased. The test was much more difficult than I expected (probably other people as well). There were questions that I would never would have expected to be on the exam, which might have been the ungraded ones.
Darril gibson's book was my primary study guide for the exam, and I read it thoroughly twice (which I highly recommend). My advice for future security plus exam takers is to have Darrils book as the priority study guide, with other supplemental guides such as Sybexs' version. You have to understand the material thoroughly in order pass the test without having a heart attack like I had, and not spent too much time on the hundreds of study questions that are out there (I personally don't find it to be too effective and redundant). Many of the questions require critical thinking to narrow down to the best answer. Wow...Long rant. I'm done.