I'm not certain that the arrogance of Microsoft is preferable to the arrogance of Oracle (what's the difference between Larry Ellison and god? God doesn't think he's Larry Ellison.). But free is a hard argument.
Just remember we can migrate your enterprise calendars (Key word: Enterprise-- if you have 100 Oracle calendars, just use client-side methods. If you have 500+ users and mission-critical meetings, check us out).