The massive advantage that iostreams offer is safety. printf() is an inherently unsafe function. Not just that, but it's trivial to overload << for your own types, whereas it's realistically impossible to extend printf()- this has the added advantage of instantly overloading for output to files as well, and anything else connected to a stream. In combination, these make printf() unusable in high-quality code. I've also noticed absolutely no performance difference, although I see many people posting about how slow they are.
