I cheer what he had done as well.
My dad was in the Army during WWII - ended up in Europe just after the Battle of the Bulge, so he was fortunate enough to be involved in a somewhat less deadly time (as compared to, say, the Normandy invasion). There is no doubt that had the US not had and used the Atom bomb, he would have been involved in the invasion of Japan. Hundreds of thousands would have died for no good reason. The bombs saved the lives of many, American and Japanese.
Truman did the right thing.
In fact, during the Vietnam War he was in favor of using the bomb on North Vietnam.
Truman's big mistake in 1945 was letting the Soviets have half of Korea as an occupation zone.
Considering the death toll on Okinawa, I think the official estimates of how many Americans and Japanese would have died if Japan had been invaded are probably considerable underestimates. The combined death toll of Japanese soldiers and civilians on Okinawa (April-June 1945) may have been higher than the death toll from Hiroshima and Nagasaki combined.