I'll be posting some of my own thoughts on Aperture 3 soon, maybe tomorrow. But I just noticed David Riecks has some issues with how Apple Aperture 3 writes metadata and I recommend you read his article very carefully indeed:
Apple has made some significant changes to how Aperture handles metadata with this latest release. However, the ways in which this has been done should be of great concern to professional photographers that work with other programs, or hand off their metadata-rich files to others who need to be able to access the full range of that information.
You shouldn't be concerned if you use Aperture 3 on your own Mac, don't typically use embedded metadata, and don't share your images with others, or work with other programs such as Adobe Photoshop. Even if you do use metadata to describe your images, and only need to find them on your local computer, you should be fine.
You should be concerned if you use Aperture to write metadata to files you use with other programs, share with others or share on the Internet. For example if you do additional work in Photoshop or Lightroom, and then archive your images using other programs, you need to understand what is happening, or risk having some or all of your metadata disappear.
Good DAM practices aren't just about only ever using any one program, or placing all your trust in one company, even Apple.