Important warning about iCloud drive:
PSA: Do not upgrade to iCloud Drive during iOS 8 installation.
ETA: according to the comments on the article, there is a compatibility problem only if you use iCloud to sync documents with your Mac, or with other iOS devices that can't upgrade to iOS8. Syncing of contacts, calendars, notes are not affected by that. So it's a bit hard to say whether I am affected or not. It's a bit confusing.
Anyway, I'll wait a few days at least before upgrading, as usual.