The email service works fairly well as you can get at it via IMAP, POP, or Webmail. Outlook's IMAP implementation is not as strong as it's Exchange support is.
However, Calendaring is more like an beta product. It is not as stable as the email program, nor feature complete. For example you can't use the Calendar service via a web calendar service, nor via WebDAV. You also can't centrally manage calendar sharing privileges. They are done via the iCal program with the calendar setup on that computer. Apple's Blog and Wiki software have no integration into the Calendar service!
Also, iCal with CalDAV has some major limitations on sending invites to users not in your LDAP Directory; iCal normally let's you invite anyone you can email.