What do you like best?
Office 365 is great for a Microsoft-centered application suite. The online portal gives you a place to administer nearly every aspect from creating and managing user email addresses, collaborative groups and distribution lists, searching and archiving inboxes, managing enterprise spam settings, and more.
It's a time saver to be able to go into a web portal, create a new user, assign them an email address, the entire Office suite, and additional programs like Visio and Project all from one page. No more managing your own on premise server, performing maintenance, and worrying about backups.
What do you dislike?
It's a Microsoft product which means it's prone to stupid little inconsistencies and annoyances that will quite possibly never be fixed or will perhaps be fixed, but just for awhile until it breaks again.
Want to download a pst file from a recent criteria based email search? You'll need a Microsoft browser to do that. Want to automatically buy a Visio license for a user when you assign it? Function doesn't work...6 months later - it works!!...3 weeks after that - it's broken again.
Also there have been a number of instances where I feel the task I want to accomplish should be easily performed through the portal, but there just isn't a way to do it and I have to fire up a PowerShell session to get it done.
Recommendations to others considering the product
Be absolutely sure you know what you want when you choose your onmicrosoft domain name, because there is no way to edit that! You're only option is to create a new one and migrate everything to it.
What business problems are you solving with the product? What benefits have you realized?
Office 365 is great for taking the load of your IT team by eliminating having to fudge around with their own server, maintain it, back it up, and troubleshoot it.
There is also an impressively powerful Office 365 Admin app for smartphones that notifies you of reported issues and let's you do a number of tasks related to user management from the convenience of your phone which is great for severing access for a no-heads up termination while you're out to lunch.