| There are many schools of thought on this and depending on which path you choose to follow you can come to a different conclusion.
1) The argument that the bombing was necessary otherwise Japan would have continued fighting is somewhat contentious as from late 1944 onwards the Japanese were attempting to negotiate a peace (or at least to determine terms for peace) via the Russian government however this wasn't in the best interests of Russian expansion and thus met with stalling tactics from the beginning.
Therefore had Stalin been thinking of anyone but his own glorification it is possible (albeit unlikely) Japan could have been out of the war before Germany and the A-Bombs would not have been used at all.
2) Based on what the West knew of the peace opportunities (which was bugger all) and on the how the Japanese had fought up to that stage it is fair to say that an invasion of mainland Japan would have been a bloody affair both in terms of Allied and Japanese civilian casualties possibly in the realms of hundreds of thousands if not millions and given that knowledge dropping the bombs probably ended up saving countless lives.
Personally given the information available at the time (which obviously did not include the long term effects of radiation sickness) I believe that the bombing was the right decision.
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We are more often treacherous through weakness than through calculation. ~Francois De La Rochefoucauld |