I recently used iPhoto Library Manager to merge two iPhoto 5 libraries of several thousand images apiece. I did this in preparation for migration to iPhoto 6; IPLM has a longer track record with iP 5 than 6, so I chose to merge before upgrading.
Thus far it seems to have gone well, which is pretty impressive since #$!$! Apple doesn't support Library imports or merges in $!@#$!@ iPhoto. I'm still looking for problems and I'll report on my experience soon. I did get some malrotated images, but I expected that. OS X had some bad bugs with EXIF headers that Apple never admitted to but fixed; I expected older images to malrotate on import. I reverted to original and fixed them.
One image among thousands failed to import. IPLM produced a handy error message. I tracked down the image -- it was corrupt. Preview and GraphicConverter wouldn't render it and if I tried to open it in iPhoto the app locked up. (How stupid is that?)
Unnerving. I expect this image went bad years ago. Could have been a disk crash, could have been a rude copy of iPhoto, could have been a network copy glitch -- heck, it could have been cosmic rays. My regular backups only go back a few months, so they wouldn't help. I do have some CDs and DVDs that would probably cover this problem, but it would take a while to track those down. Fortunately I happened to have older versions of some iPhoto Libraries on my external drive, and I found a good copy in minutes.
This is exceedingly annoying. We are so far from having really good backup solutions. I depend on redundancy -- a primary automated network backup that runs daily and several different approaches to backup that are done irregularly. Odd are that if one method fails, the very different "backup backup" methods will work. It's the same principle applied to designing the space shuttle's control system.
Backups have saved me at least a dozen times over the past 20 years or so. They're very demanding to manage however.