Apple quickly put out a patch for Aperture 3 to improve stability on a great many things including:
• Upgrading libraries from earlier versions of Aperture
• Importing libraries from iPhoto
• Importing photos directly from a camera
• Memory usage when processing heavily-retouched photos
• Face recognition processing
I myself had repeated troubles on an iPhoto import, so fingers crossed on my current attempt, with newly patched software.
It's clear that everyone with Aperture 3 should absolutely download this update. However, support documents still warn that the thing could eat up your drive (albeit temporarily):
Aperture temporarily uses extra space on your hard drive during the upgrade process. This is for the purpose of backing up critical library info and insuring the integrity of your data during upgrade. If you don't have adequate space on your hard drive to accommodate the upgrade, Aperture will display a warning dialog. You may need to move your library to a different hard drive with more space in order to upgrade it, and then move it back to the original drive when the upgrade is complete. Any space used by Aperture during the upgrade is released and made available to you again once the upgrade has finished.