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From http://support.apple.com/kb/TS2518:

This update improves overall stability and addresses a number of issues in Aperture 3, including (abridged):

• All the features we added with this new version…
• Some existing features that didn’t really work all that good anymore with the new version…
• and much more.

And, before I forget, does it make a difference? It did for me, most notably, my loading time was improved and browsing through the pictures was snappier.

I have some remaining face detection running and its using a fair chunk of cpu (130%+). Also just having Aperture sit idle takes up about 850mb+ memory.