We moved from a locally hosted exchange, to O365. Please note: my experience is that of helping the end user configuration only, I didn't assist in any back end implementation. Moving to O365 was less painful than I had anticipated, but still came with its own set of problems. We use Outlook 2010 (Windows) and Outlook 2011 (Mac) as the front end, as well as a web interface. The web interface changed with O365 (being that it is now hosted on outlook.com) but other functionality remained the same. I must, therefore, write about the differences between the two experiences and list the hurdles we had. Firstly, with the move to O365, we no longer had a system which was hidden in our intranet. Anyone anywhere could access it, so we lost a degree of control (which should be expected when moving to third party) over access. Mainly, this meant our policy to install a mobile device management profile to allow access to mail was pretty much killed. Users can now install the outlook app on their phone and download all their emails to their device, something that should give companies a pause for thought. This hasn't caused any big problems so far, and so, I am hopeful. Secondly, accounts which had no password (certain resource accounts) now needed a password. A minor inconvenience. Finally, it also had the effect of needing a reconfigure of third party software (such as Spiceworks ticketing/inventory software) to SMTP rather than exchange. I would try to use exchange, and it appeared to work with some software, but it would not. Obviously, this is more a limitation of the third party software, but it is something you may want to be aware of. All in all, it wasn't bad, but the lower level of control is an ongoing concern to me and is only somewhat offset by the reduced support needed internally.