Stephen Hackett, in a good piece on the significance of today's iOS 7 release:
What’s really crazy is that Apple unveiled iOS just three months ago. It’s been a mere 100 days since developers saw the new UI, got their hands on the new physics engine and first encountered iOS 7’s font changes.
In addition to updates, today marks the introduction of many new apps to the iOS App Store. These apps are almost all iOS 7-only. Clearly developers believe that if they take advantage of some of the new technology available today, they may be able to take a foothold in what may have been a market previously locked-up by a competitor.
Their work has paid off. Today, iOS 7 shines not only because of the work put into it by Apple employees, but the army of third-party developers who make their living on Apple’s platforms.
I've had the GM build of iOS 7 on my 4S the last few days, and I love it. I can't wait to get my 5S and see iOS 7 on a bigger 4-inch canvas. The optimized-for-7 apps I've seen update on my phone look fantastic. Today officially marks the beginning of a new era; it's a breath of fresh air.
See also: Marco Arment's advice for warming up to the new-look software.