When I took the Commercial written, I passed with a 97%. I don't feel it was wasted effort because it made my oral exam quite easy. The examiner figured I knew my regs, and just quizzed me on my knowledge of the aircraft and the aerodynamics of the maneuvers. I second getting the Gleim book. I studied it endlessly for a week, and it really helped. On some of the difficult calculation questions, I knew the answer without calculating it because I had seen that problem/answer so many times.

Good luck!