I taught both before and after formal training and I think it did make a major difference. There's no teacher like experience, however.
Also, some of the 'formal training' can be of a really poor standard.
Agreed. Formal training helps, but, do you really need it? This reminds me of the age-old argument - education vs. experience. The teacher that has been in the classroom for 10 years vs. the newbie that just started fresh with his or her shiny new MA in TESOL. Which is better?