Instant awake from sleep on PowerBooks and iBooks. Seems like a small thing, but gosh I love it, especially at work. On my AirPort-connected PowerBook, I have access to dozens of radio stations from A to Z, not to mention my own MP3 collection. It's really easy. Now, when I awaken my PowerBook in a new location, it automatically finds the AirPort connection.
Some applications run slower.
I can already tell that I much prefer Cocoa apps to Carbonized ones. It's expensive to upgrade the right way. I'm having a hard time getting my OS X Mac to communicate with my OS 9.1 Macs. My previously 4-hour PowerBook battery now only lasts about two and a half hours. So, after reading these pros and cons, if you've decided you're ready to make the leap, here's what to do.