MACZENTRISCH  Try it, and tell me ... :-)

At the moment I just know of problems with certain images (with included XMP-data) using Leopard.

With these images the changes are not saved and you get an error message instead.

I hope to have a new beta ready next week, which should also significantly improve the workflow.  
(Version 1.0b8)

MACZENTRISCH  At the moment shootShifter only uses the EXIF-creationdate ("date and time of original data generation") as the EXIF-datesource.

Images that only have an EXIF-modificationdate ("file date and time") are treated like images without EXIF-tag. And these are ignored with the default settings.

You can try the option "use finderdate as datesource" in the Preferences instead. But it will only change the finderdates and not the EXIF-modificationdate of these images.

The next beta should improve this ... i hope to have it ready soon.  
(Version 1.0b8)