I haven't used this method, but just want to add my 2 cents. I would be cautious about using it to avoid. Many women (myself included) can have patches of fertile quality cervical fluid when they are not in fact fertile, which can lead you to believ that your fertile days have ended (when they haven't yet shown up!) Does that make sense?
Let me give a more specific example, using, of course, me. My cycles since coming off of the pill have been 31 days long. Last cycle I O'ed around day 19. This cycle I had fertile quality fluid on CD 13, but by day 14 it seemed to get dryer, and today (CD 15) too. If I was going on that alone, it would seem to indicate that I O'ed on CD 13 and then started "drying up." But because of my previous cycle and my temps, I know that I'm actually still waiting to O.
I guess if you are extremely familiar with your cycle, or tend to have "textbook" fertility signs, it would probably work. But if it was me, I'd still use a back-up method.
Hope you find something that works well for you.