It's perfect on the iPhone 5 and is probably the OS it should always have come with.
From what I hear 7.1 dramatically improves things in the iPhone 4 too.
Well it's crap on my i5, I waited six months for even worse battery life. Gee thanks Apple.
As for 'dramatically improving the iPhone 4', well it doesn't. iOS7 dramatically ruined the i4, and iOS7.1 merely improves matters slightly. Performance is still WAY worse than iOS6 by a massive margin.