Not an IT professional but we went through it about 6 months ago. 16,000 employee company.
Really not that big of an issue and very little down time. My biggest complaint, and a lot of it is due to my IT Dept, is that almost 3-4 days a week when I first log in after getting to work, I will have multiple updates. This is not a big deal other than many of the updates take 30 minutes to complete.
I went in last week and got there at 7:00 to knock some stuff out before everyone else got in and my computer updated until 7:45. Completely wasted me getting there early.
Pretty painless migration and my IT department is not very good.