In Pictures: App wars - Windows 8 Metro vs. the iP...
Calendar: Metro loses
For basics, Metro's Calendar is fine: You can work with multiple calendars and invite people to events (for accounts that support this feature), but new invites are not so easy to see -- there's no invite inbox that ensures you can see them all as in the iPad's calendar.
As is typical to the basic Metro UI of using menulike bars for most controls, basic operations such as switching calendar views and adding events require extra steps.
In Pictures: App wars - Windows 8 Metro vs. the iPad