For those of you who have to discuss this with friends/relatives/co-workers, etc., I'd like to give you a few talking points.
1. The most effective argument for the use of the bomb is, as stated above, it saved millions of lives. Most of these would have been Japanese, Chinese, et al., not Allied soldiers. So if someone states that the bombs were a bad idea, ask them "What method of ending the war quickly do you propose and what method would result in fewer casualties?" The idea of blockade is often put forward, but that's also called "starving them out." We all know who gets the most and best food when there is a siege, the military and the command structure. The old and the young are usually last, and as they were already stressed in Japan, a starvation campaign would have been devastating. Add to this the thought that it's hardly the moral high ground here as it's indiscriminate and would have affected the entire population, not just two cities.
2. It was going to get worse. If the shock and awe campaign hadn't worked, Gen. George Marshall had asked that the next eight bombs be reserved for combat deployment during Operation Olympic. So there would have been ten places nuked in Japan. And the proposed deployment plan was for the Allied troops to go through "Ground Zero" about 1/2 hour after the bomb was dropped. The casualties due to radiation exposure might have caused the invasion to grind to a halt as the front line troops were followed through the blast area by the support units. Being ignorant of the real dangers of "fallout" we might have set up hospitals for the radiation cases in "hot" areas, hastening their demise. And the Japanese civilians would either not be getting any treatment or they would be swamping the medics.
