We are working on the tail end of a migration from on premise resources (Exchange, File server, etc) to completely "cloud" ie O365. Mail, domain, AD, all has gone very smoothly. File server too ...

Plenty of organizations have moved from on-prem file shares to SharePoint online. If you are expecting the web version of "Office" (Word, Excel, etc.) to work for everything, its not there yet.

It too can work with the major ECM systems, including SharePoint, IBM Content Manager, Open Text Livelink and EMC. Watch out for further solutions to whip all that unruly content into shape!