Aperture 1.5.3 addresses issues related to overall reliability and performance in a number of areas, including:

Generation of thumbnails for adjusted images
Entering and exiting Full Screen mode
Working with large sets of keywords in the Keywords HUD
Restoring from a vault
Among the specific issues that have been addressed:
Previews now update properly when images are sent to an external editor.
Leaf Aptus 22 and Aptus 75 images are now imported with the correct orientation.
When folders are imported as projects, the folder structure is now correctly preserved when identically named subfolders are included in the hierarchy.
Reconnecting referenced images that have been externally edited now works more reliably.
Setting the ColorSync profile in the Aperture Print dialog now correctly suppresses color management settings in the Mac OS X Print dialog.