There have been threads about this movie before, e.g.,
http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=439&topic_id=763731While I agree with W. C. Green that the movie is worth watching, I'd like to point out that like most movies set in ancient times, this one perpetuates a myth and should not be confused with a search for historical truth.
It is a myth that the dark ages were ushered in by the destruction of the Great Library and the consequent loss of ancient knowledge. This mythical event has been blamed variously on Pagans, Christians and Moslems. One version of the myth has the fire occurring in 48 BCE as a result of the burning of Julius Caesar's fleet. Another version (propagated by this movie) has it occurring in 391 CE. In yet another version, "... the Great Library of Alexandria was destroyed by the Arabs after their conquest of the city in 642 A.D., by order of the Caliph Umar."
http://books.google.com/books?id=hNtcejI_IGkC&pg=PA213&lpg=PA213&dq=myth+library+alexandria+umar&source=bl&ots=KKBWPwL4eY&sig=HGKftswcTdGKM5RdpBUnnJNnVOI&hl=en&ei=xSWnTbi1OJP4sAPIvsn6DA&sa=X&oi=book_result&ct=result&resnum=7&ved=0CD4Q6AEwBg#v=onepage&q=myth%20library%20alexandria%20umar&f=false None of these versions of the myth holds up under scrutiny by historians.
The movie "Agora" also tries to create a brand new myth: that Hypatia anticipated Kepler's theory of elliptical planetary orbits. There is nothing in the historical record to suggest that she did any such thing.
But don't let my criticism stop you from enjoying the movie!