Climategate: as with the recent MPs' expenses debacle, it's taken leaked information to uncover the true extent of the corruption involved. In both instances the leaking of the data helped to circumvent any stage-managed release of the requested information and any massaging of the figures before they were officially published under FoI laws by the authorities involved (or by the Information Commissioner).
In the case of the Climate Change data, it is unlikely that such information would have seen the light of day had it been left to the FoI authorities to release it. This raises the question of just how effective FoI actually is - if people in public authorities want to hide information from public disclosure they will find ways to do so. Or they will find ways to manipulate the release of the information so as to try and minimise any adverse publicity.
For example, the Scottish MPs' expenses were released under FoI - there was some (limited) evidence of MPs claiming for expenses they were not entitled to ("taxi for McLetchie") but no suggestion of widespread fraud and corruption. By contrast, the UK MPs' expenses were publicly disclosed before the information was filtered through the official FoI disclosure route - the scale of fraud and corruption revealed was staggering. It makes you wonder.