Fibonnacci is numerology. So far no one has been able to produce the slightest sensible theory to explain why fib numbers should crop up in stocks (though I have some suspicions how it might be done).
On the other hand, like most TA I use it all the time. Mostly because traders are such a superstitious bunch that anything that becomes popular is bound to start having an effect on the market as traders start using it for turning points. And Fib is the most common of such superstitions.
Having said that, I can show you a very good and simple explanation as to why markets retract 50% so often.
