My organization recently migrated to Office365, including Exchange / Outlook 365.
I was impressed with Outlook 365 OWA (outlook.office365.com). My initial thought was that having the entire infrastructure presented through a single portal is a recipe for disaster. Yet as soon as I type in the @company.com portion of my email address, it re-directs to our own authentication infrastructure (Ping in our case). None the less, I am sure that there are people working night and day trying to figure out how to MitM outlook.office365.com
Office 365 OWA and Outlook 2013 are nearly identical in terms of UI layout and functionality. For the average user, I think that they could do without the desktop client and most likely, not notice much if any loss of functionality.
Clutter is working well. I turned it on almost two weeks ago. So far it has done a great job of filtering out of all of the junk emails from the sales drones, while at the same time letting the important emails through. I have not missed any emails that I need to see.