While many people would argue that there were cons to this decision, I would argue that most of the cons were overridden by the pros.
One con that some might offer is the fact that many people in Hiroshima and Nagasaki were killed by the bombs. I would argue, however, that many more Japanese would have been killed if it were not for the bombs. If the US had had to invade the Japanese home islands, the devastation would have been worse. The US would surely have bombed all major cities in Japan incessantly, killing more people than the atomic bombs did. The US would then have invaded and tremendous numbers of people would have been killed in the fighting.
Another con is that it unleashed nuclear weapons on the world. However, it seems obvious that nuclear weapons would have come about even if the US had not used them right then. It may, in fact, be better that they were used when they were relatively weak rather than later when hydrogen bombs and other more powerful weapons had been developed.
The major pro is that the decision ended the war quickly. This saved countless numbers of lives, both American and Japanese. The US military estimated that it would face 1 million casualties if it had to invade. Casualties on the Japanese side would surely have been even greater.
The dropping of the atomic bombs was a terrible thing, but an invasion would have been even more terrible.