Goes to show you, 6+ weeks of planning pays off!
80 desktops, law firm that are HEAVY email users. Senior management would not allow for any downtime longer than about an hour. 800GB Exchange 2010 database. Used Migrationwiz Premium. Dirsync. And since there were over 1500 messages larger than O365's 25MB limit, I used nirsoft.net's FREE nifty "OutlookAttachView" program.
I'll point out at this time that there are MANY blog posts and even Technet articles that provide conflicting information. Did I mention I did 99% of this remotely? :)
Life is good.