Just installed it on my G5 Powermac, and ... could it be that Apple has finally got Mail's Smart Mailboxes working reliably? My life revolves around this feature, but it drives me nuts because it doesn't reliably isolate the right messages.
First impression was that they've cracked it, but ... no, I still get random stuff until I do a Get Mail on another Smart Mailbox.
Have to confess I've signed up here to see how Expression Engine handles your site. So far, looks good!