I just completed a complete "Migration" from BE10d to BE11d. I have a total of 15 MMS and 1 CAS server. My upgrade was conducted over a 2-week period setting up a new CAS and copying all my policies and selection list over to the new CAS. Overall my experience was great, I ran Full System Backups yesterday and issues that I had in the BE10d environment are no longer an issue. Symantec has done a very good job with version.
My recommendation to you is plan-plan-plan, I decided to have a parallel environment in the event that things went south.
From what I was told from Symantec about the differences is that BE10d did not scale very well, and with the engineering of BE11d it scales much better. My major issues in BE10d was duplicate job failures, I can tell you that active jobs are smoother.
Issues I'm currently having post migration is Media Management, Reporting on "Recently Written Media" and Vault Rules. This is all part of the pre planning, if you do this correctly you should have a successful upgrade.
Hope this helps
Eric Reid