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. So, after reading these pros and cons, if you've decided you're ready to make the leap, here's what to do. Figure out the equipment that you need to buy, beg, or borrow, and wrestle with your budget.