Ironically, that's almost what happened in my case. I took the PPI online test review (which was truly atrocious), but had foot surgery in January, so didn't really start studying until the last week in February when I went back to work. Since I had been off work so long and had to play catch-up, I didn't get to study much, and also had to teach a 40 hour class. I ended up spending most of my time tabbing my book out instead of doing problems, but on exam day, I found that the tabs were worthless, so I did my entire Thermal Fluids ME exam by using the MERM index that I hadn't looked at until exam day... still passed, though!
Lesson learned? I should have photocopied the index and put it in a binder and taken the NCEES Practice Test instead of spending lots of time and money on the prep course.