We've been using OS X Server for about 5 years. The only component we utilize is the calendar. We have 26 users with calendars, and we use resources like conference rooms, projectors, etc. Lately, the calendar has been causing issues. Resources are difficult to manage/reserve, and some users have completely lost their delegates randomly. I'm tasked with investigating some alternative calendar solutions. We currently use the basic email service offered by Rackspace. They're value added resellers for Google Apps and Office 265 in addition to offering Hosted Exchange, so those are my obvious choices. We currently use Google Docs for a lot of text documents, spreadsheets, and slide decks, so Google Apps for Business might be a natural fit. I don't have any experience with the Google calendar in a business setting, but I know many businesses use it. Exchange would work since we already have all the Office desktop apps. Unfortunately, we're not up to date with those apps, so Office 365 might be an even better option. Microsoft seems pretty dominant in the group calendar space, but I don't have any personal experience with it. Anyone care to share your thoughts on these options?