The immense majority will experience no issue, as it would be expected that any bug that creates data loss would be a release blocker and, had it been verified during development, it would have been hunted down implacably.

Chances are it didn't manifest itself before.

For what I know of Steve's reputation, I wouldn't like to be in the iPhoto team right now.

But then, I would always keep the photos as separate files on a directory tree with the metadata in a single file. This way, the chances of a catastrophic data loss would be very slim.