How it works: Voice synthesis on the Mac has been around for more than 20 years now, but outside of screenreading and special applications it's never been a marquee feature (that is, if you don't count Talking Moose -- now once again available for OS X. Woot!). Even though the Macintalk and Speech Manager voices have improved a lot over time, the best of them still sound distressingly artificial.
Enter Alex, a dramatically more comprehensible voice (YouTube demo movie) that includes fine breath and pause control to enhance understandability, especially for high-speed reading; you can listen to a sample of Alex at this post. From the samples of Alex that I've heard, he compares favorably with high-end synthesis voices like AT&T's Natural Voice and Cepstral. For anyone using VoiceOver or wanting to be able to track information while not watching the computer, Alex is a natural.
Who will use it: VoiceOver users, who'll be thrilled; all Leopard owners who occasionally need things read instead of displayed.