You make a good point about corruption, as there clearly is a problem. There have been several scandals (oil-for-food and the Congo sex scandal immediately come to mind). This tells me that there needs to be an attempt at serious reform so that there is better oversight/checks and balances. The UN has outlined several steps to try this reform.
http://www.un.org/reform/mgmt_reform.shtml#1What jumped out at me is that an independent commission has been formed (IAAC) by bringing in 5 auditors from outside the UN that roughly represent most of the geography of the world. They also will adopt more transparent accounting practices and reporting of functions.
At this point, these are just words, but I am willing to give the reforms a few year's chance before writing off the UN completely. The IAAC has only been in existence for a few years. This stuff should have been done a long time ago, but at least its being attempted.