Using Mac OS X Server e-mail and calendering

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.

Mail service is great. No problems on any client machines so far, but then IMAP is old enough now that you wouldn't expect huge compatibility issues. Exchange isn't supported yet, so if you need that feature set, you're out of luck. Webmail works fine, but is uninspiring:

Mail service details here: http://images.apple.com/server/macosx/docs/L355775B_MailSvcs_tb.pdf

As for calendering - it works great in Mac OS Mail.app. I haven't tried accessing calendaring from a Windows box but I understand it should work with Outlook 2007 - I suspect it won't be as smooth an experience with your Windows users compared to Exchange though. It uses CalDev protocol so it works with Mozilla Sunbird etc. You can create shared calendars but it does rely a fair amount on Directory service - some of the coolest features don't work for users who aren't bound to LDAP.