I think the only thing that you can say about the attack on Hiroshima was that it _did_ end the war, not that it was the only way it could have ended or that it was even necessary. So I'd say: sufficient. The decision of 'unconditional surrender' was pretty well made before the attack on Nagasaki. Gar Alperovitz has spent a career building a (IMHO, pretty good) case that it wasn't necessary, but I don't think there's any discussion whether it was sufficient (even within what I've read of Hasegawa's work on the subject). Gar's book is great, but if you don't have the time to read a 1000 page book, check out this site; I think it answers a lot of what the current thought about all those issues is.
http://www.doug-long.com/debate.htm
For instance, the "shock value of the Soviet entry into the war" (which was Truman's stated reason for going to Potsdam) is discussed at length.
/jordan