The fond memories Mbeki evokes nowadays boggels my mind. Both Mbeki and Zuma ran terrible presidencies. The nature of their flaws differ though. 

As president, Mbeki was neurotic, autocratic and vain. His Aids policy was murderous, he endorsed Mugabe's stealing of elections, manipulation of state resources for party political ends was pioneered by him and the arms deal happened on his watch. BUT, he did seem to think he was acting in the best interest of South Africa and there is no evidence that he was personally corrupt.

Zuma's presidency has only two objectives: avoiding jail and personal enrichment. He has hollowed out every institution that stands in the way of these objectives. Zuma is totally disinterested in the wellbeing of the country. 

South Africa doesn't need another incompetent ANC leader. The ANC has had enough time to show what they can do - with dire results. Time to give another party (or coalition of parties) a chance to rule.