With Mac OS X 10.6 Server, Apple has made significant improvements to their server OS. Major new features like Address Book Server and Mobile Access Server complement the solid improvements in existing services. The addition of Push services for e-mail and calendaring finally allow Apple to more properly support its own iPhone product.
Ironically, Apple is so far ahead of most everyone else in supporting things like CalDAV and CardDAV that trying to use non-Apple clients with iCal and Address Book server is a mess of unfriendly clients, or random third party plugins to Outlook et al, and unfortunately, Apple isn’t much help for Mac OS X 10.6 Server administrators trying to deal with this problem. Hopefully that will improve over time, but for anyone supporting non-Mac clients, keep in mind that you may have some interesting times ahead of you with Mac OS X 10.6 Server.
From what I’ve played with, it’s a solid upgrade.