John Siracusa has an in-depth review of Mac OS X 10.5.1 I was holding off upgrading to Leopard until I read that 10.5 significantly improves the performance of older machines.

Apple is legendary for OS rollouts that are faster than their predecessors. This phenomenon is known as "teh snappy" The upgrade from 10.3 (Panther) to 10.4 (Tiger) didn't provide much more by way of teh snappy, but Leopard does. Where YouTube videos on my 1.33 GHz 12" iBook G4 (1 GB RAM) would stutter and jerk, Leopard renders them smoothly on the same machine, not to mention Spotlight's newfound alacrity and the Finder's continued responsiveness (even in the face of unavailable network volumes).

Leopard's improvements to Mac OS X are too many to mention here. This is not to say Leopard is all good news. For one, it is impossible to unmount remote and local drives once they've been used.2

About a month ago, I went to the Apple Store in Columbus, Ohio and purchased an iPod Touch. After using that device for two days I returned it and purchased an iPhone in its place. Since then, I've been telling anyone who cares to listen (and some who don't) that iPhone is the first practical step toward truly ubiquitous mobile computing. The applications and services which are going to be developed for iPhone and (coming) devices with comparable functionality will significantly evolve computing. In my opinion, truly mobile computing is going the be the next big thing whose effect on computing will be comparable to the effect of the debut of Macintosh in 1984.

Among the things Leopard updates is Setup Assistant's welcome movie (which I poached from the bottom of the "Background" section of Siracusa's review.)

As a bonus: those of you who have an iPhone and who know how to add custom ringtones can download the ringtone I made from the first fifteen seconds of Leopard's intro movie.3

Happy upgrading.