Outlook is, dare I say it, damn useful. Integrating a calendar with email is so obvious and so useful that I've often wondered why the concept hasn't been cloned into open source yet (and before you say it, I don't have the time to do it myself or the leadership skills to assemble a team to help).
This is the biggest fault with how outlook is designed. You don't want mail and calendar to be the same app. They should integrate with each other but let a calendar app do calendars well and a mail client mail well. For sure invitations etc should integrate just as it does with Apple products. But hey having calender in the mail app is my biggest complaint about Outlooks UI.