Hiroshima is one of the most evocative places I have ever visited. The hardest thing with Hiroshima is separating right and wrong. At times, the museum glosses over Japan crimes in WWII - as is true through-out the whole country.
Whatever the other arguements the site is incredibly moving and a compelling arguement against nuclear weapons.
But, I would ask all visitors to say an extra pray at the monument for the Korean victims of the bomb. These people, slave labourers transported from Korea, have been ignored by Japan for far too long - the Governments lack of apology on this matter is inexcusible.
The final thing that I must add about Hiroshima is that the people of the city lift the spirit beyond belief. It is, perhaps, Japans most friendly city.