I am not sure how common serotonin syndrome is. According to the Wikipedia article, it occurs in 14-16% of patients who overdose on SSRIs. However, I'm not clear on its definition of 'overdose.'

I currently take Effexor and Prozac and have for at least four years. I have had no problems. Effexor is in the same category as Cymbalta and Prozac is in the same category as Paxil.

You can find the Wikipedia article on serotonin syndrome here.