Fedora 12 officially released a few days ago. Today I downloaded the latest bits and installed it. Then I went through the typical process of installing the programs I use and updating all the packages.
If you want to use the Nvidia or Fusion Nvidia drivers there are some new steps involved with installing the driver. For details visit the Fedora Forum
I found that with the Fusion driver my system was very sluggish compared to Fedora 11. When I use the Fedora Nouveau driver I lose GLX direct rendering.
My first impression is that Fedora 12 did not obtain thorough testing for this release.
For now, I will continue to use Fedora 11. Your mileage may vary.