Now feeling fast is a mighty spiffy trick, and mostly boils down to the way they double buffer the windows and use VSYNC to good effect.
To be honest, I've used MacOS X and wasn't impressed. It's great demoware, and a good OS, but nothing we can't beat the living snot out of in free software land given a bit of work. Anyway, unless you will enjoy writing Yet-Another-Cocoa-Instant-Messenger, there's not much to do there except wait and see what Apple does next. Hardly thrilling stuff.