I think when the research conducted by revisionist historians of the Vietnam War is considered, it becomes clear that Diem was actually a fairly capable leader who I think has been unfairly demonized by orthodox historians in the past.

Orthodox historians of the Vietnam such as Stanley Karnow and David Halberstam portray Diem as a corrupt American puppet that antagonized his people. However, this is not entirely true.

Recent revisionist research has shown that Diem was more effective that he is generally given credit for. Mark Moyar in Triumph Forsaken: The Vietnam War 1954-1965 (2006) argues that Diem was a wise and effective leader who could have led South Vietnam to victory over the communists. In essence, he argues that the American decision to overthrow and murder Diem in 1963 was one of the greatest mistakes of the United States in Vietnam. His argument relies on a reevaluation of Diem's domestic policy, his military record, and the actual motives of the Buddhist protesters.

Moyar argues that Diem's nation building policies can be likened to land reform and that they were effective at keeping the countryside out of the hands of communist influence. Professor Philip Catton of Stephen F. Austin State University has also written favorably about Diem's nation building policies.

Furthermore, Moyar argues that Diem was an effective military leader and that by 1963 ARVN had become very skilled at fighting the Viet Cong and that they were on the road to victory, a development that stopped with the American backed overthrow of Diem in 1963.

Moreover, Moyar and some other revisionists argue that Diem was effective and that the Buddhist protesters had ulterior motives that went beyond protests against oppression. Rather, they argue that the Buddhist protesters opposed Diem because they desired political power. Arthur Dommen was also among the first to note that the Buddhist protesters, whose charges of religious oppression crippled the South Vietnamese government from 1963 to 1965, had fabricated evidence of oppression and were more concerned with gaining political power than religious freedom. Moyar goes even further by arguing that the Buddhist movement had been infiltrated by communists and that Diem was justified in his opposition to the Buddhist Movement for this very reason.

From these points, I think it is clear that Diem was far from the corrupt tyrant that the orthodox historians portray him as. He achieved real success during his years as South Vietnam's president, both socially and militarily, and presented a genuinely viable alternative to the communist People's Republic of Vietnam. If the United States had not intervened and helped overthrow Diem, the Republic of Vietnam may have had a better chance at victory over the communist opposition forces.

However, while I generally agree with the revisionist historians of the Vietnam War that Diem was largely an effective leader, I think that their arguments need to be taken with a grain of salt. Moyar especially is very committed to undoing the myth of Diem that had been propagated by the orthodox historians that I think he goes to the opposite extreme. Instead of arguing that Diem was a horrible leader, Moyar paints Diem as a brilliant leader and the best leader that the RVN could have had.

Yet, this approach riddles his book with limitations. Moyar presents weak evidence about the communist involvement with the Buddhist protesters, ignores or downplays genuine evidence of Viet Cong strategic successes from 1961-1963, and ignores the numerous failures of Diem's nation building policies. Furthermore, Moyar argues that because Vietnam lacked democracy in its past, Diem's authoritarianism was necessary to govern (which I feel is somewhat incorrect and too simplistic of a justification for some of Diem's more oppressive policies).

Despite the numerous flaws in Moyar's approach to Diem, I feel that he is still generally correct. Diem was a capable leader and deserves more credit that he is generally given. Even today, the orthodox interpretations of the Vietnam War are still more popular than their revisionist counterparts.

Thus, to wrap up my answer to your question, yes, Diem was generally a good leader and he was not the corrupt tyrant that many make him out to be. While he may not have been the infallible leader that Moyar makes him out to be, I think that the revisionist interpretation of Diem is worth looking at more so than the orthodox argument because it provides genuine examples of the successes that Diem achieved during his time as president of the RVN. However, the limitations of this school of the historiography of the Vietnam War must still be considered to get a fuller understanding of Diem.

Regardless, I think that new research clearly show that Diem, while not perfect, was generally an effective leader and not simply an oppressive tyrant.

Sources:

Books:

Triumph Forsaken: The Vietnam War 1954-1965 by Mark Moyar

Vietnam: A History by Stanley Karnow

Other:

Vietnam: Historians at War by Mark Moyar (http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1042&context=usarmyresearch)

Vietnam’s Changing Historiography: Ngo Dinh Diem and America’s Leadership by Derek Shidler (http://castle.eiu.edu/historia/archives/2009/Historia2009Shidler.pdf)

Revisionism with a Vengeance: A Review of Mark Moyar’s Triumph Forsaken: The Vietnam War, 1954-1965 by Edward Miller (http://www.shafr.org/passport/2007/december/Miller.pdf)