For me, the Pre-NAPLEX was not a "preparation", but more of a possible indication of how I would do on the real thing. I made sure I was very familiar with the material before I signed up and took the Pre-NAPLEX online, just so I could see how well I would do on it. (They give a range of scores where you may fall if you take it many times.)
I got my score for the NAPLEX, and it was within 10 points of the Pre-NAPLEX that I took. It might be different for others, though. But it did relieve a lot of stress I had regarding the NAPLEX because I could see what kind of questions were being asked, and the format of the questions.
I'm sorry you guys had such a bad experience, though. And good luck on the NAPLEX if you haven't taken it yet!