Was Reconstruction a failure or success?

One of the more disturbing observations that one can make about United States history with respect to the outcomes of the Civil War and the period of Reconstruction was the fact that institutionalized racial segregation and the struggle among African Americans for civil rights survived more than halfway into the 20th century. Legislation granting basic rights to African Americans was still being debated in the United States Congress a century after the end of the Civil War. That is a failure of Reconstruction.
Reconstruction was a process whereby the South, physically and mentally defeated, would essentially be rebuilt physically and politically to better mirror the North. It was a process of reuniting two very disparate entities. The war cemented the South’s unification with the North. Reconstruction was needed to maintain that unity by culturally and politically transforming the vestiges of the South—an effort that could be termed, to employ a World War II vernacular, as "a bridge too far." Reconstruction succeeded insofar as the adoption of state constitutions and legislatures more in-tune with the post-war mandate at eliminating slavery and incorporating newly-freed slaves into the nation’s fiber was achieved. Politically, in other words, Reconstruction could be viewed as a success. It was, in the broader sense, however, a failure in that the cultural transformations required to bring the South into alignment with the North did not occur. In fact, the notion of “carpetbagging” was born to symbolize a sense among Southerners of Northern imperialism being imposed upon a proud culture, however morally defective that culture remained regarding racial equality.
Because the South continued to resist, often militaristically in the form of white supremacist terrorists like the Ku Klux Klan, and politically in the form of Jim Crow laws, the advance of the United States as a nation more representative of the ideals upon which it was founded would be delayed for many tumultuous decades. That was a failure of Reconstruction.

On the whole, you'd have to say it was a failure. The whole purpose of Reconstruction was to change the structure of Southern politics and society so that the slave states could be reincorporated into the Union. The nation as a whole would become stronger, reducing the chances of another civil war, and the newly won constitutional rights of African Americans would be guaranteed. Despite an initial flurry of activity by Reconstruction's advocates, most of these objectives weren't achieved in the long run, especially the last one.
The main problem was that the policy dealt with the formal elements of Southern political life—representation in state legislatures, voting rights, and so on—instead of tackling the substantive issues that held back African Americans, such as white supremacy and a widespread belief in the inequality of the races. In other words, Reconstruction dealt with formal, rather than substantive equality. Although a number of African Americans were allowed to vote and elected to state legislatures for the very first time, the institutions of government still remained firmly in the hands of those implacably opposed to racial equality and civil rights.
A further fatal flaw with Reconstruction was that it was overly dependent on the concerted political will of the federal government to make it work. Inevitably, this was difficult to sustain for any appreciable length of time. Interest in Reconstruction waned substantially from the 1870s onwards, so that eventually, even a narrow, formalistic approach to the policy was abandoned. The abandonment of Reconstruction had a disastrous effect upon African Americans in the South, giving rise as it did to the construction of a whole new apparatus of legalized racial oppression, reducing former slaves to a status not too dissimilar from what they had endured prior to the passing of the Thirteenth Amendment.