I hope their opt-out is granular enough that I can still use the parts of iCloud that don't make me uncomfortable.
Personally, I find that Apple has been very consistent with how they treat customer data at a corporate level and I trust their incentive to protect it and/or hide it from public view by default WAY more than I trust Googles, Amazons, and especially FaceBooks. If I'm honest with myself their vested interests in iAd do raise the hairs on the back of my neck a little bit, but it would be a significant departure (and would create more backlash than anything I can imagine them doing in the near term) from everything I've seen of their treatment of personal data to date.
What you get after the set-up, in Settings, looks a bit like this: http://i.imgur.com/dYXvU.png