One of the main speakers at Mathfest, 2011, was one of the Fields Medalists this year: Manjur Bhargava, of Princeton.
Here is what I said about him on the first day:
Lastly Manjul Bhargava of Princeton (who is already full professor though he is less than half my age; he was an Andrew Wiles student) gave a delightful lecture on algebraic curves.
What I noted: all three of these mathematicians are successful enough to be arrogant (especially the third). They could have blown us all away. Yes, they took the time and care to give presentations that actually taught us something.
Some top-of-the-line researchers are excellent teachers too.