There have been reports of a few concerns with the upgrade process on this site, and while I may eventually get to respond to them individually, for the moment I wanted to share a very useful link on apple.com and some general tips on making this update as painless as possible.
First and foremost—make no mistake, this is a HUGE upgrade to your library. A lot has to happen for the library to be completely “version 3” ready. The library structure itself has changed, which is what allows you to do things like switch between libraries without relaunching. There’s a new RAW engine, so the images have to be re-processed by that (that can happen later though) before the new features like Curves and Brushes are available. The images all have to be processed for Faces (again, this can be deferred) and that in itself is incredibly labor intensive. The master files must be polled for EXIF data to look for GPS info, but that is pretty quick.
There’s an invaluable article on apple.com that you should read before upgrading if you have a large library—detailed explanations of what I’ve just stated above in there, clear as a bell.