So, it's out. It took all night but I got it onto my iPhone 4S last night. I have to say, it's lovely to use. The flat style works very well on a relatively small screen in your hand, even if I still don't like that Safari icon.
Forgetting the style though, what I really like are the many small improvements to the stuff I do every day. The camera app is much faster and has finally caught up with Instagram, etc; Safari supports more than eight tabs; the UI when making a phone call is much better (I do a lot of conference calls so this is invaluable); texting is slicker; the calendar works better; home screen folders now have sub-pages so they're almost like mini-home screens; transitions are universally better (including stealing Android's lovely fade when turning the screen off/on the notification bar is finally truly useful; and basically there are just improvements to the ease-of-use and presentation of information everywhere.
So yeah, apart from the style it's definitely a case of iteration over big new features, but I would argue that end-to-end refinement of the UI is something that's been needed for years now.